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Full text of "Subsurface geologic methods, a symposium"



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SUBSURFACE GEOLOGIC 



(A Symposium) 
Second Printing — Second Edition 

Compiled and Edited by 

L. W. LeROY 

Associate Professor of Geotogy, Colorado School of Mines 




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Single Copy $7.00 
COLORADO SCHOOL OF MINES 

Department of Publications 

Golden, Colorado 

1951 



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Copyright 1950 by the Colorado School of Mines. All 
rights reserved. This book or any part thereof may not be 
reproduced in any form without the written permission of 
the Colorado School of Mines. 

First Edition, June 1, 1949. 

First Printing, Second Edition, June 1, 1950. 

Second Printing, Second Edition, June 1, 1951. 



Printed by Peerless Printing Co., 
Denver, Colorado. 

Engravings by Cocks-Clark Engraving Co., 
Denver, Colorado. 



CONTRIBUTORS TO SUBSURFACE GEOLOGIC METHODS 

L. W. LeRoy, Associate Professor of Geology, Colorado School of Mines, 

Golden, Colorado, Compiler and Editor 
George W. Johnson, Department of English, Colorado School of Mines, Golden, 

Colorado, Editor 
0. E. Barstow, Dowell Incorporated, Tulsa, Oklahoma 
C. M. Bryant, Dowell Incorporated, Tulsa, Oklahoma 
John G. Caran, Core Engineer, San Antonio, Texas 
Kirk Carlsten, Mechanical Engineer, Eastman Oil Well Survey Company, 

Denver, Colorado 
S. R. B. Cooke, Department of Metallurgy and Mineral Dressing, University 

of Minnesota, Minneapolis, Minnesota 
James G. Crawford, Chemical and Geological Laboratories, Casper, Wyoming 
Jack De Ment, De Ment Laboratories, Portland, Oregon 
H. G. Doll, Schlumberger Well Surveying Corporation, Houston, Texas 
George H. Fancher, Department of Petroleum Engineering, University of 

Texas, Austin, Texas 
R. D. Ford, Schlumberger Well Surveying Corporation, Houston, Texas 
M. G. Frey, Department of Geology, University of Cincinnati, Cincinnati, Ohio 
John W. Gabelman, Colorado Fuel and Iron Corporation, Pueblo, Colorado 
R. J. Gill, Geologic Survey Company, Wichita, Kansas 

R. G. Hamilton, Schlumberger Well Surveying Corporation, Houston, Texas 
P. N. Hardin, Dowell Incorporated, Tulsa, Oklahoma 

W. E. Hassebroek, Halliburton Oil Well Cementing Company, Duncan, Okla- 
homa 
Sigurd Kermit Herness, Department of Geology, Colorado School of Mines, 

Golden, Colorado 
John M. Hills, Consulting Geologist, Midland, Texas 

H. A. Ireland, Department of Geology, University of Kansas, Lawrence, Kansas 
J. Harlan Johnson, Department of Geology, Colorado School of Mines, Golden, 

Colorado 
F. Walker Johnson, Creole Petroleum Corporation, Maracaibo, Venezuela 
Paul F. Kerr, Department of Geology, Columbia University, New York, New 

York 
Truman H. Kuhn, Department of Geology, Colorado School of Mines, Golden, 

Colorado 
J. L. Kulp, Department of Geology, Columbia University, New York, New York 
H. L. Landua, Humble Oil & Refining Company, Houston, Texas 
Arthur Langton, Baroid Sales Division, Los Angeles, California 
Julian W. Low, The California Company, Denver, Colorado 
V. J. Mercier, Mountain Iron and Supply Company, Wichita, Kansas 
Carl A. Moore, Department of Geology, University of Oklahoma, Norman, 

Oklahoma 
J. B. Murdoch, Jr., Eastman Oil Well Survey Company, Denver, Colorado 
P. B. Nichols, Geolograph Company, Incorporated, Oklahoma City, Oklahoma 
W. D. Owsley, Halliburton Oil Well Cementing Company, Duncan, Oklahoma 
L. L. Payne, Hughes Tool Company, Houston, Texas 

Gordon Rittenhouse, Department of Geology, University of Cincinnati, Cin- 
cinnati, Ohio 
George L. Robb, United States Bureau of Reclamation, Denver, Colorado 
N. Cyril Schieltz, United States Bureau of Reclamation, Denver, Colorado 



G. Frederick Shepard, General American Oil Company of Texas, Dallas, 

Texas 
L. L. Sloss, Department of Geology, Northwestern University, Evanston, Illinois 
W. Alan Stewart, Department of Geology, Colorado School of Mines, Golden, 

Colorado 
Harrison E. Stommel, Department of Geophysics, Colorado School of Mines, 

Golden, Colorado 
E. F. Stratton, Schlumberger Oil Well Surveying Corporation, Houston, Texas 
H. F. Sutter, Baroid Sales Division, National Lead Company, Houston, Texas 
Wilfred Tapper, Halliburton Oil Well Cementing Company, Duncan, Oklahoma 
John D. Todd, Consulting Petroleum Geologist, Houston, Texas 
Paul D. Torrey, Lynes Incorporated, Houston, Texas 
R, Maurice Tripp, Consulting Engineer, South Lincoln, Massachusetts 
Warren R. Wagner, Department of Geology, Colorado School of Mines, Golden, 

Colorado 
W. A. Wallace, Halliburton Oil Well Cementing Company, Houston, Texas 



PREFACE TO THE FIRST EDITION 

During the past fifteen years numerous papers pertaining to methods 
applied in deciphering subsurface geologic conditions have appeared in periodi- 
cals such as World Oil, the Oil and Gas Journal, the Petroleum World, the 
Bulletin of the American Association of Petroleum Geologists, the Bulletin of 
the Geological Society of America, the Journal of Paleontology, the Journal of 
Sedimentary Petrology, the Transactions of the American Geophysical Union 
and publications of the American Institute of Mining and Metallurgical Engi- 
neers. Commercial organizations have issued in pamphlet form discussions of 
techniques in the field of subsurface geology that have been highly enlightening. 

Such text and reference books as Petroleum Production Engineering 
(McGraw-Hill) by Uren, Manual of Sedimentary Petrography (Appleton- 
Century) by Krumbein and Petti John, Methods of Study of Sediments (Mc- 
Graw-Hill) by Twenhofel and Tyler, Sedimentary Petrography (Nordeman) by 
Milner, Sedimentary Rocks (Harper) by Pettijohn and Examination of F rag- 
mental Rocks (Stanford University Press) by Tickell treat to a degree certain 
methods applicable to the establishment of subsurface values. 

The need for a publication that would bring together information relating 
to subsurface geologic techniques that has hitherto been unavailable or scat- 
tered has long been realized by geologists and petroleum engineers. An attempt 
to satisfy this need is undertaken in the present text. Subsurface Geologic 
Methods. Had it not been for the liberal cooperation and interest of the con- 
tributors to this volume, such a compilation would not have been possible. 
Special recognition and appreciation are extended to those contributing sections 
of this text and to the organizations of which they are a part. 

Many illustrations used in this symposium have been obtained from pub- 
lished sources, credit being given for each. 

The section entitled "Multiple-Differential Thermal Analysis" has been re- 
printed by permission of the American Mineralogist; those entitled "Induc- 
tion Logging" and "Dipmeter Surveys" are by permission of the American 
Institute of Mining and Metallurgical Engineers. The sections "Deep-Well 
Camera" and "Character of Pores in Oil Sand" are reprinted through the 
courtesy of World Oil. 

Plans are to revise the symposium periodically as new methods are intro- 
duced or as the methods discussed are modified. Certain topics in this com- 
pilation may not be treated as completely as some individuals may wish; those 
desiring further detail should refer to the articles and publications cited. Criti- 
cisms and suggestions will be gratefully received by the editor, and these will 
be considered for subsequent editions. 

It is hoped that Subsurface Geologic Methods will assist geologists and 
petroleum engineers in the field as well as educational institutions that offer 
formal courses in subsurface geology. 

June 1, 1949 



PREFACE TO THE SECOND EDITION 

The first edition of Subsurface Geologic Methods, published in June of 
1949, had a gratifying reception as a textbook for universities offering courses in 
subsurface geology. Owing to the unexpected rapid depletion of the supply of 
this volume, it was deemed necessary to revise and enlarge the book as a second 
edition. 

The first printing of second edition, published in June, 1950, includes sev- 
eral new sections which are of concern either directly or indirectly to the 
subsurface geologist. These additions cover the following subjects: secondary- 
recovery methods, evaluation of petroleum properties, geochemical methods, 
micrologging, drill-stem testing, mud chemistry, cementing problems, acidiza- 
tion, shale-density analysis and graphic methods in mining. A series of ques- 
tions has been added at the termination of each chapter to permit improved 
instruction in university work. An index has also been included in this volume. 

Grateful acknowledgment is hereby given to the Department of Publica- 
tions of the Colorado School of Mines for making possible the publication of 
this volume and to George W. Johnson, Acting Director of Publications, for 
his efforts and interest in the editing of the present volume and for supervising 
the numerous engraving and binding problems. 

Special acknowledgment is given to the American Association of Petroleum 
Geologists for permission to reprint John M. Hills' paper, "Sampling and Ex- 
amination of Well Cuttings," and to the American Institute of Mining and 
Metallurgical Engineers for the courtesy of permitting the publication of H. G. 
Doll's paper, "The Microlog." 

Acknowledgment is also given William F. Dukes, student at the Colorado 
School of Mines, for the excellent and accurate drafting of 162 illustrations in 
both the first and second editions. 

To former President Ben H. Parker of the Colorado School of Mines sin- 
cere appreciation is extended for his wholehearted support of this symposium. 

I am greatly indebted to the contributors of this volume whose cooperation 
has made Subsurface Geologic Methods possible. 

The demand for Subsurface Geologic Methods has exhausted the present 
supply and it has become necessary to issue a second printing. 
June 1, 1951 L. W. LeRoy 



CONTENTS* 
Chapter Page 

1. Introduction by L. W. LeRoy 1 

2. Stratigraphic, Structural, and Correlation Considerations 

by L. r. LeRoy 12 

Unconformities by W. Alan Stewart 32 

3. Comments on Sedimentary Rocks by L. W. LeRoy 71 

4. Subsurface Laboratory Methods 84 

Micropaleontologic Analysis by L. W. LeRoy 84 

Calcareous Algae by J. Harlan Johnson 95 

Detrital Mineralogy by Gordon Rittenhouse 116 

Insoluble Residues by H. A. Ireland 140 

Petrofabric Analysis by Warren R. Wagner. 157 

Micro (Petrographic) Analysis by Warren R. Wagner 

and John W. Gabelman 172 

Size Analysis by L. W. LeRoy 184 

Settling Analysis by L. W. LeRoy 193 

Stain Analysis by L. W. LeRoy 193 

Shape Analysis by L. W. LeRoy 199 

Electron-Microscope Analysis by Carl A. Moore 202 

X-Ray Analysis by N. Cyril Schieltz 211 

Multiple-Differential Thermal Analysis by Paul F. Kerr 

and /. L. Kulp 240 

Water Analysis by James G. Crawford 272 

Core Analysis by John G. Caran 295 

Fluoroanalysis in Petroleum Exploration by Jack De Ment 320 

Shale Density Analysis by F. Walker Johnson 329 

5. Subsurface Logging Methods 344 

Sampling and Examination of Well Cuttings by John M. Hills.— 344 

Electric Logging by E. F. Stratton and R. D. Ford 364 

Induction Logging and Its Application to Logging of Wells 

Drilled with Oil-Base Mud by H. G. Doll 393 

The Microlog by H. G. Doll 399 

Radioactivity Well Logging by V. J. Mercier 419 

Caliper and Temperature Logging by Wilfred Tapper 439 

Well Logging by Drilling-Mud and Cuttings Analysis 

by Arthur Langton 449 

Drilling-Time Logging by G. Frederick Shepard 455 

Driller's Logging by L. W. LeRoy 475 

Drilling-Time Logging fey P. B. Nichols 478 

Spectrochemical Sample Logging by L. L. Sloss and 

5. R. B. Cooke 487 

Composite-Cuttings-Analysis Logging by R. J. Gill 495 

6. Miscellaneous Subsurface Methods 504 

' Controlled Directional Drilling by J. B. Murdoch, Jr 504 

Oil-Well Surveying by J. B. Murdoch, Jr 548 

Oriented Cores by Kirk Carlsten 591 

* An index of authors and a subject index are included at the back of this volume. 



CONTENTS— Continued 

Chapter Page 

6. Miscellaneous Subsurface Methods — Continued 

Magnetic Core Orientation by M. G. Frey 596 

Coring Techniques and Applications by H. L. Landua 609 

Application of Dipmeter Surveys by E. F. Stratton and 

R. G. Hamilton 625 

Design and Application of Rock Bits by L. L. Payne 643 

Deep-Well Camera by O. E. Barstow and C. M. Bryant 664 

The Electric Pilot in Selective Acidizing, Permeability 

Determinations, and Water Locating by P. N. Hardin 676 

The Porosity and Permeability of Clastic Sediments and 

Rocks by George H. Fancher 685 

Drilling Fluid Chemistry by H. F. Sutter 713 

Hydrafrac Treatment by W. E. Hassebroek 723 

Formation Testing by W. A. Wallace 731 

Oil-Well Cementing by W. D. Owsley 746 

Well Acidization by L. W. LeRoy 750 

Geochemical Methods by R. Maurice Tripp 760 

7. Secondary Recovery of Petroleum by Paul D. Torrey 775 

8. Valuation and Subsurface Geology by John D. Todd 792 

9. Duties and Reports of the Subsurface Geologist 

by George W\ Johnson 810 

10. Graphic Representations by L. W. LeRoy 856 

11. Subsurface Maps and Illustrations by Julian W. Low 894 

12. Subsurface Methods as Applied in Mining Geology 

by Truman H. Kuhn 969 

13. Subsurface and Office Representation in Mining Geology 

by Sigurd Kermit Herness 989 

14. Subsurface Methods as Applied in Geophysics 

by Harrison E. Stommel .!..1038 

15. Geologic Techniques in Civil Engineering by George L. Robb 1120 

16. Sources of Well Information 1150 



CHAPTER 1 
INTRODUCTION 

L. W. LeROY 

Subsurface geology involves interpretation of the stratigraphic, struc- 
tural, and economic values below the earth's surface. These interpreta- 
tions are based on information obtained from bore holes, geophysical 
data, and projected surface information. As stated by Jakosky,^ "The sub- 
surface is an extremely complex three-dimensional region, which the in- 
terpreter must diagnose with only the help of the very limited type of 
data obtained from our present techniques." 

The subsurface geologist is required to have: (1) a basic back- 
ground in geologic structure, (2) a broad and fundamental knowl- 
edge of rock types, (3) a three-dimensional concept of geologic phe- 
nomena, and (4) an understanding of the economics of the problem. He 
must be able to coordinate and accurately integrate these related phases. 
He must realize the importance of structure to stratigraphy and strati- 
graphy to structure. He must be aware that stratigraphic and structural 
problems cannot be solved without first evaluating the rocks and their 
relationships. To place rock types in their proper categories, emphasis 
must be made on the various techniques and methods which permit more 
exacting classification. Electrical, radioactive, caliper and other logging 
data cannot be accurately interpreted until the lithologic aspects of the 
strata have been established. The rocks and structures developed within 
them must be treated from the three-dimensional viewpoint because both 
change in space. Folding intensities and characteristics and lithologic 
patterns exhibit variations which must be recognized before appraising 
the geologic impress. Subsurface geology demands a creative imagination, 
an analytical and systematic approach, and a multiple-hypotheses manner 
of thinking. The geology of tomorrow depends largely on the assertive- 
ness of the subsurface geologist of today. 

Subsurface geology as applied in the petroleum industry has made 
rapid advances since 1925. In many areas it has attained greater impor- 
tance than surface geology; the discovery of most future oil fields of 
the world will undoubtedly be attributed to subsurface geologic studies. 
Oil operators today fully realize the exigency of appraising subsurface 
conditions in exploration and development programs. 

Methods applied in evaluating subsurface conditions vary in nature 
and complexity, depending on the character of the rocks penetrated, the 
type of equipment available, the quality and quantity of data desired, and 
the time allocated to the solution of the problem. 



^ Jakosky, J. J., Whither Exploration: Am. Assoc. Petroleum Geologists Bull., vol. 31, no. 7, 
p. 1121, July 1947. 



Subsurface Geologic Methods 



SUBSURFACE GEOLOGY 



St rafigrophy 



Structural 
Geology 



Geophysics 



Economics 



Petroleum Production 

Engineering Engineering 

I I Development I I 
Engineering 



Sedimentology 



Sedimentogrophy 



Petrogrophy 



Paleontology 



Detritol 
MInerology 



Figure 1. Subsurface geology and its relationship to other sciences. 

The subsurface geologist should be familiar with all techniques that 
assist in determining subsurface phenomena and should recognize limita- 
tions of these methods. 

Some 25,000 wells drilled each year add new sources of geological data 
requiring close study by specialists in petrology, paleontology, stratigraphy, 
geochemistry, geophysics, and structure who have the responsibility of deter- 
mining correct regional correlations, suitable classification of the rock se- 
quences, isopach and paleogeologic maps, faunal and floral zones, facies 
changes, vertical and lateral extent of oil- and gas-bearing zones, character 





















MANAG 


EMENT 






Su 


bsurface Geologist 






Petroleum Engineer 
1 




^ / 




\ 

Stratlgraptiy 


Electrical logging 


/ 1 

Chemistry 




Paleontology 


Radioactive •■ 


Physics 




Structural geology 


Ttiermol " 


Mathematics 




Mineralogy 


Caliper n 


Thermodynamics 




Sedimentation 


Micro li 


Fluid a gas studies 




Sedimentogrophy 


Drill-time ■• 


Mechanics 




Petrology 


Induction n 


Reservoir investigations 




Petrograptiy 


Spectrochemicol analysis 

X-ray 

Core 

Mud cutting 

Electron - microscope " 

Formation testing 

Cementing 

Acidizing 

Directional drilling 

Coring 


Strength of materials 


















1 



Figure 2. Correlation between some of the duties of the subsurface geologist and 

petroleum engineer. 



4 Subsurface Geologic Methods 

of reservoir rocks and their fluids, time and place of progressive structural 
developments as well as the present normal strike and rate of dip for each 
"layer of geology." . . . Not only is there need of a greater store of geological 
information but also of well established criteria by which such data may be 
used more effectively. Much progress has been made toward the development 
and use of methods for recognition of favorable regions, localities, and well 
locations, but more definite analysis is no doubt possible as well as highly 
desirable.2 

The subsurface geologist is scheduled to play a major role in ful- 
filling these requirements. 

In the early history of drilling, little attention was given to the de- 
tailed characteristics of subsurface conditions; as a result, many geologic 
data were lost or so imperfectly recorded that reliable interpretations are 
now difficult to make. Present-day programs require systematic and accur- 
ate subsurface recordings. These requirements have vastly improved such 
problems as (1) exact structure contouring; (2) accurate definition of 
fault patterns and their relationship to oil-producing intervals; (3) the 
location and evaluation of unconformities; (4) facies changes and thick- 
ness trends; (5) the interpretation of geophysical data; (6) the origin, 
migration, and accumulation of hydrocarbons; (7) building, bridge, and 
dam foundations and tunneling; (8) locating and outlining ore bodies; 
(9) improvement of surface drainage systems; (10) evaluation of ground- 
water patterns; and (11) interpretation of soil data. 

The decipherment of certain of these problems offers little difficulty 
and thus requires only a few techniques. Other subsurface conditions 
may be of such complexity that voluminous data are needed before logical 
and satisfactory answers become evident. 

Noble ^ recently made the following statement: 

Much of our future supply of oil will be found by close teamwork between 
various oil-finding groups, utilizing all of our present known prospecting tools 
and exploration methods. To a great extent this effort will consist of detailed 
subsurface studies in the search for stratigraphic traps and accumulations on 
the flanks and extensions of known structures; and deeper drilling wherever 
possible. There are, however, some rather extensive areas having good oil pos- 
sibilities but about which we know very little because of a cover which masks 
the underlying geological conditions and which we cannot effectively penetrate 
with any of our present methods because of physical or economic considerations. 

These masks include overthrust segments, blankets of young vol- 
canics, thick alluvial and glacial deposits, multiple unconformities, and 
even water, which covers favorable stratigraphic and structural condi- 
tions on the continental shelves. Noble points out the oil and gas possi- 
bilities underlying the thrust sheets, as at Turner Valley in western Alberta, 
and similar conditions in Montana and the Pacific Coast and Midcontinent 



^Cheney, M. G., The Geological Attack: Am. Assoc. Petroleum Geologists Bull., vol. 30, no. 7, 
pp. 1079-1080, July 1946. 

^ Noble, E. B., Geological Masks and Prejudices: Am. Assoc. Petroleum Geologists Bull., vol. 31, 
no. 7, pp. 1109-1117, July 1947. 



Introduction 5 

regions. Attention must be given the possibilities buried beneath late 
Tertiary volcanics of the Rocky Mountain and Columbia Plateau areas 
and in Central and South America. The evaluation of these problems 
rests with the subsurface geologist. 

Duties of the Subsurface Geologist 

The duties and responsibilities of the subsurface petroleum geologist 
are numerous and varied. He must be diversely trained and have an ac- 
curate sense of geologic and economic values. He must be able to present 
his data concisely, to adhere to his convictions, and to coopera.e fully 
with the petroleum, production, and development engineer, the field geolo- 
gist, the geophysicist, the management, and all other personnel that con- 
tribute to the solution of the subsurface problem. Frequently in explora- 
tion work he may be called upon to devote many continuous hours to 
special assignments; or he may be designated to obtain information from a 
"no dope" well, wherein he must investigate the casing program, the 
acidizing and shooting procedure, the record of mud, chemicals, and bit 
sales, and logging activities. 

Some Major Subsurface Problems 

Subsurface problems in the fields of petroleum exploration and 
development are varied and numerous. The solution of some of these 
problems is relatively simple, whereas, the solution of others demands 
voluminous data that must be carefully screened and integrated. Some 
of the more important problems commonly encountered in subsurface 
work are as follows: 

1. Correlation of Surface to Subsurface Stratigraphic Units: It 
has been demonstrated that both recent and ancient deposits 
of the stable shelf areas frequently are lithologically and faun- 
ally in discord with deposits of the unstable shelf. The intra- 
cratonic basin sediments and their organic elements vary widely 
with those that accumulated under geosynclinal conditions. Thus, 
wells drilled in a geosynclinal facies penetrate sections which 
require correlation with equivalent though lithologically and 
faunally dissimilar marginal strata. The establishment of cor- 
relations of this type are essential before the tectonic and 
sedimentational history of the region can be properly evaluated. 

2. Reef Investigations: During the past few years considerable 
attention has been given reef production in Texas and Canada. 
Fanatical attempts have been made to improve and devise new 
methods applicable to the discovery of gas and petroleum res- 
ervoirs of this type. Seismograph exploration has been extremely 
instrumental in many of the reef production discoveries. De- 
tailed lithologic, paleontologic, and well-logging data must be 
coordinated in reef investigations, as the reef elements (porosity, 



Subsurface Geologic Methods 

permeability, composition, texture) change rapidly both vertic- 
ally and horizontally. Reef trends and development are not con- 
stant; thus, it is necessary cautiously to outline drilling and leas- 
ing programs. Acidizing, shooting, and formation testing are 
critical subsurface problems in reef production problems. 

3. Secondary Recovery: Oil companies today are concerned more 
than ever before with the problem of obtaining greater yield of 
oil and gas per acre. Repressuring, water flooding, acidizing 
and shooting of wells, directional drilling, proper water shut- 
offs, systematization of drilling, proper well spacing and con- 
trolled production are of major concern in secondary recovery 
programs. The subsurface geologist and engineer must com- 
bine their efforts to secure best results in secondary recovery 
problems. 

4. Interpretation of Well-logging Data: Since 1930 the electrical 
log has been very successful in evaluating features of the pene- 
trated strata. Since this date radioactive, caliper, thermal, drill- 
time, induction and micrologging have been introduced into 
subsurface investigations. Information obtained from these 
logs is based on the characteristics of the rocks — their composi- 
tion, texture, and fluid and gas content. Many profile anomalies 
cannot be adequately explained; thus, more attempt should be 
made to evaluate these idiosyncrasies by detailed analysis. 

5. Acidizing and Shooting: What interval to acidize and to shoot 
is a problem of major concern to operators drilling in carbonate 
sections. Before either or both methods are initiated, the lith- 
ologic and structural aspects of the strata should be adequately 
known and this information integrated into other logging data. 

6. Prediction of Drilling Difficulties: The oil operator and con- 
tractor are vitally interested in knowing the difficulties and 
magnitude of difficulties prior to commencement of drilling. A 
sandstone-shale section would present different problems than 
a carbonate section or a section containing numerous beds of 
salinifereous material. A rock sequence containing considerable 
bentonite might alter an entire drilling and casing program. 
Other problems may be encountered during penetration of fault 
surfaces, unconformities and vuggulated strata in which circula- 
tion could not be maintained. 

7. Improved Subsurface Mapping: Subsurface data may be con- 
veniently represented by contour-type maps. These maps are 
based on structural, isopachous, isochor, isothermal, isosperm, 
isochron, isopotential, lithofacies, and depth pressure informa- 
tion. Viscosity, fluid and gas density maps may also be pre- 
pared. Paleogeologic data in many areas are commonly plotted 



Introduction 7 

and shown in map form. All such maps permit an improved 
understanding of subsurface conditions. 

8. Unconformities : The character, extent of erosional surfaces, and 
the relationship of such surfaces to adjacent strata are often 
much improved by subsurface information. These surfaces 
must be accurately defined before stratigraphic and economic 
values can be evaluated. 

9. Onlap and Off lap: Onlap and off lap problems require the three- 
dimensional approach. Subsurface studies permit determination 
of rate, dimension, and trend of these depositional conditions. 

10. Miscellaneous Problems: Other subsurface problems confront- 
ing the geologist and engineer in addition to those mentioned 
above include: cementing, setting of casing, hole caving, fishing, 
stabilization of drilling fluids, perforating, formational water 
variations, porosity and permeability changes, coring, and test- 
ing. 

Training of the Subsurface Geologist 

Courses in subsurface geology, as given in some universities in the 
United States, vary according to geographic location, facilities, and instruc- 
tional personnel. Prior to these courses the student should have a thor- 
ough background in petrology, petrography, structural geology, field 
geology, petroleum geology, mineralogy, stratigraphy, sedimentation, 
paleontology, and geophysics. 

A sequence of subject material in a formal university course in sub- 
surface geology is suggested. 

Lithologic Studies: Lithologic types including shales, limestones, 
dolomites, sandstones, and other lithologic varieties should be examined 
and studied under the binocular microscope. Each lithology should be 
represented by chip fragments in a reservoir-type slide and viewed by 
the student during instruction. This method is extremely applicable and 
time-saving in preparing the student for subsequent well-logging assign- 
ments. 

Well-Logging Methods: The various types of well-logging methods 
should be briefly summarized. These methods should include lithologic, 
electric, radioactive, drill-time, caliper, and thermal logging. The instru- 
mentation, use, and limitations of these methods should be treated. 

Theoretical Electrical Profile Interpretation: After the student has 
become familiar with lithologic and electrical relationships, he should be 
required to plot from tabulated data (mimeographed) a percentage log 
from which an interpretive log is prepared. On the basis of the latter 
log theoretical electric profiles may be drawn. This problem demands 
that the student think in terms of both lithology and its probable electrical 
reflection. 

Preparation of Well Log: The student, once having become familiar 



8 Subsurface Geologic Methods 

with the basic lithologic varieties and logging methods, should be assigned 
a well section to log. This work involves the examination and plotting of 
a lithologic log from ditch cuttings and cores. The lithology should first 
be plotted on a percentage basis, and subsequently the lithic boundaries 
should be adjusted and an interpretive log prepared. Color symbols 
should be used to represent lithologies (pi. 11). In addition to interpret- 
ing and recording the lithologic sequence of a well, each student should 
have available an electric-log profile with which to make comparisons. 
Upon completion of the log a final report of the well should be required. 

Contouring: Several electric-log series from oil fields should be 
studied and correlated, and subsurface structural and isopachous maps 
prepared. Cross sections should also be incorporated. In addition to this 
problem, a number of theoretical contour problems should periodically 
be submitted to the class to improve structural interpretation. 

Correlation: Paired electric, radioactive, and lithologic logs from 
various fields should be correlated in detail, and the results carefully 
drafted and discussed. 

Principles of Stratigraphy and Correlation: The fundamental con- 
cepts of stratigraphy and correlation should be treated and should include 
such topics as facies changes, unconformities, onlaps and offlaps, and any 
other criteria that control the correlation of strata. 

Laboratory Methods: The various techniques followed in subsurface 
or stratigraphic laboratories should be broadly outlined and reviewed and 
should include such topics as detrital mineralogy, insoluble residues, stain 
tests, micropaleontology, and thin-section, screen, and sedimentation an- 
alyses. 

Miscellaneous Topics: Such topics as directional drilling, acidizing, 
cementing, secondary recovery, formation testing, and coring should be 
briefly reviewed with emphasis placed on the geologic aspects. 

In the graduate school more specialized subsurface problems should 
be emphasized. The subject matter should include structure contouring, 
fault problems, correlation interpretation, paleogeologic and lithofacies 
mapping, isopachous studies, and the preparation of subsurface geologic 
reports. These courses should be presented with the intention of intro- 
ducing to the student the "work pressure" factor that prevails in eco- 
nomic programs. 

Those students having a geologic-engineering background are fa- 
vorably adapted for subsurface geologic investigation. This does not 
imply that such training is essential to develop good subsurface person- 
nel; it cannot be denied, however, that the basic sciences of mathematics, 
physics, chemistry, hydraulics, thermodynamics, and descriptive geometry 
provide favorable attributes. 

To train an individual for subsurface geology as applied today in 
the petroleum industry should require at least eight years after termina- 
tion of hia advanced academic work. A minimum of two. years in surface 



Introduction 9 

geologic mapping is a necessity. Subsurface phenomena would be diffi- 
cult to interpret adequately without exposed stratigraphic and structural 
relationships having first been observed and studied. The student of 
subsurface geology should spend at least three years in stratigraphic 
and paleontologic laboratories in order to become familiar with funda- 
mental techniques applied in subsurface problems. This period of training 
should be divided among the Pacific Coast, the Gulf Coast, and the Mid- 
continent areas. One year should be devoted to geophysical work, another 
to logging methods, and the last to petroleum and production engineering. 

Future of Subsurface Geology 

Most of the oil fields and ore deposits in the future will be discovered 
from the application and coordination of subsurface investigations. 

The number and complexity of subsurface methods now employed 
require specialists. There are those who devote their efforts to lithologic 
studies, to paleontologic investigations, and to logging methods, and 
others to interpretation of structural conditions. The field of geophysics 
offers an unlimited opportunity for the subsurface geologist whose responsi- 
bilities are to synchronize electrical and gravity data with stratigraphy and 
structure. It is essential that each specialist know the position and rela- 
tionship of his field to the general subsurface problem involved. 

Subsurface geologic methods and approaches as employed by the 
petroleum industry have not yet been fully accepted or utilized by the 
mining industry in exploration and exploitation. In the early history of 
mining only the surface or near-surface deposits were exploited and de- 
veloped. Today it is required that possibilities of deeper ore concentra- 
tions be considered. Cram'* comments: "Mineral geologists must be 
permitted by the mineral industry to spread their wings on a full-time 
basis, not on a consulting basis, if the nation's undiscovered reserve of 
minerals is to be developed." 

Subsurface investigations are scheduled to occupy an important posi- 
tion in engineering geology. Before a civil engineer can properly design 
any structure, he should be versed in the materials upon which his struc- 
ture is to rest or in which his work is to be carried out. In any engineering 
project, preliminary investigations are required and should be undertaken 
with two objectives in view: (1) the evaluation of subsurface conditions 
at and in the immediate vicinity of the proposed site that may affect the 
work program, these conditions generally involving local geologic struc- 
ture and distribution and character of undergound water; and (2) the 
determination of the character of rock types expected during the progress 
of construction. 

Many projects involving tunnels, excavation, earth movements, 
bridge, dam, and building foundations, and water supply require the 

^ Cram, I. R., Geology Is Useful: Am. Assoc. Petroleum Geologists Bull., vol. 32 no. 1, pp. 7-8, 
Jan. 1948. 



10 



Subsurface Geologic Methods 



1927 1928 1929 1930 1931 1933 1934 1935 I93S 1944 1945 1946 1947 1948 



16.2<»S 16,655 16,6 



17,823 17,696 



Figure 4. Drilling-depth records from 1927 to 1949. (Modified from World Oil.) 

services of a geologist who has the ability to coordinate and evaluate sub- 
surface data. 

1927 Chansler-Canfield-Midway Oil Company's Olinda 96, Olinda, California. 

1928 Texon Oil and Land Company's University 1-B, Big Lake, west Texas. 

1929 Shell Oil Company's Nesa 1, Long Beach, California. 

1930 Standard Oil Company of California's Mascot 1, Midway, California. 

1931 Penn-Mex Fuel Company's Jardin 35, State of Vera Cruz, Mexico. 

1933 North Kettleman Oil & Gas Company's (later taken over by Union Oil Com- 
pany) Lillis-Welch 1, Kettleman Hills, Kern County, California. 

1934 General Petroleum Corporation's Berry 1, Belridge, Kern County, California. 

1935 Gulf Oil Corporation's McElroy 103, Gulf-McElroy, Upton County, west Texas. 
1938 Continental Oil Company's KCLA-2, Wasco, Kern County, California. 

1944 Standard Oil Company of California's KCL 20-13, South Coles Levee, Kern 
County, California. 

1945 Phillips Petroleum Company's Schoeps 3, wildcat, Brazos County, Gulf Coast, 
Texas. 

1946 Pacific Western Oil Corporation's National Royalties 1, Miramonte area, Kern 
County, California. 

1947 Superior Oil Company of California's Weller 51-11, wildcat, Caddo County, 
Oklahoma. 

1948 Standard Oil Company of California's Maxwell 1, Ventura County, California. 

1949 Superior Oil Company's Pacific Creek Unit 1, Sublette County, Wyoming. 

Comparative Use of Some Subsurface Techniques 

At present, lithologic- and electric-logging methods are most exten- 
sively used in correlating the substrata. Radioactive and drill-time logging 
are becoming increasingly important and will play a greater role in future 
subsurface evaluations. Radioactive logging (gamma and neutron) has 
proved its dependability in limestone and dolomite sections by its ability 
to indicate porous strata which indirectly reflect possible petroliferous 
zones. Controlled mechanical means of recording penetration rates have 
greatly enhanced the value of the drill-time log and have placed data of 
this category on a firm basis. 

Of the micropaleontologic criteria employed in correlating the sub- 



Introduction 



11 



strata, the use of Foraminifera is the most widely applied, with that of 
ostracods second in application. Insoluble-residue work is adaptable 
locally and has been of major assistance in correlating carbonate sections. 
Detrital mineralogy has been minimized in stratal correlations; locally, 
however, it has its value. Table 1 indicates the relative usage of the more 
common methods of correlating subsurface strata in various areas. 



TABLE 1 

Relative Importance of the More Common Methods Used 
IN Correlating Subsurface Strata in Various Areas 













Paleontology 












(U 










X 




g 


•^ 60 


? fen 


1) 

•1 60 


•S 






^1 








"3 60 


rs 60 




2 




♦5 C 








e 60 


C 60 


o 


to 




<u-| 




M 


f<^ 


cc-S 


C)-2 


fa. 


o 


^ t; 


Cl g 


Pacific Coast 


2-3 
3 




3 
3 


5 
5 


1 
1 


2-3 




.s 


Gulf Coast 


5 


Permian Basin 


1 




3 


2 


1 




2 




Midcontinent (Kansas, 


















Oklahoma) 


1 




3 


2-3 


2 




4 




Illinois Basin 


1 




2-3 


2-3 




4 


3 




Rocky Mountain region 


2 




3 


6 






6 


5 


Western Venezuela 


1-2 




4 


4 


2 






2 


Eastern Venezuela 


1-2 




4 


4 


2 








Burma 


2 
2 






5 


1 
1 








Trinidad, B. W. I 




Central Sumatra 


2 


2-3 






1 


3 







Questions 

1. Define "subsurface geology." 

2. Why has it been necessary to improve subsurface geologic prac- 
tices? 

3. Subsurface methods vary in nature and complexity. Why? 

4. Present-day petroleum-exploration programs require systematic 
and accurate subsurface interpretation. These requirements have 
improved what problems? 

5. What are some of the essential duties of the subsurface geologist? 

6. Review table 1 which concerns the relative importance of meth- 
ods used for correlating strata in the subsurface. 

7. Compare the drilling records from 1927 to 1949. 

8. What is meant by "geologic masks"? Give several examples. 



CHAPTER 2 

STRATIGRAPHIC, STRUCTURAL, AND CORRELATION 
CONSIDERATIONS 

L. W. LeROY 

Stratigraphy incorporates the study of the character, sequence, rela- 
tionship, distribution, and origin of sedimentary rocks. As expressed by 
Kay,^ "Stratigraphy is veritably the interpretation of the record of pro- 
gressive movements (crustal) evidenced in sedimentation." Stratigraphy 
constitutes the basis of correlation, and correlations are primary requisites 
for surface and subsurface mapping and for the evaluation of structural 
and sedimentational patterns. 

The basic principles of stratigraphic geology have remained more 
or less unmodified, although methods and techniques applied in the solu- 
tion and presentation of stratigraphic problems have greatly improved 
during the last decade. In recent years emphasis has been placed on 
graphic representation of stratigraphic data. The construction of litho- 
facies, isopachous, log, paleogeographic, paleogeomorphic, paleoclimato- 
logic, paleogeologic, and palinspastic maps has contributed enormously in 
outlining stratigraphic trends and has introduced new avenues of strati- 
graphic approach. The recent contributions by Krumbein,^ and Dapples, 
Krumbein, and Sloss ^ ^ are examples of new lines of thought in strati- 
graphic compilation. Block diagrams are being used frequently in order 
to illustrate third-dimensional effects and to assist nongeologic personnel 
better to understand stratigraphic concepts. Such diagrams are particularly 
useful in preparing regional reconnaissance reports. 

Large sums of money are being spent annually by oil companies on 
stratigraphic research and in the training of specialists in such fields as 
micropaleontology, lithology, detrital mineralogy, insoluble residue, and 
well logging. Some companies prefer training individuals who will sub- 
sequently devote their major efforts to restricted phases of stratigraphic 
geology; other companies favor personnel familiar with diversified strati- 
graphic procedures. 

Many mineral concentrations occur in sedimentary rocks. They may 
be of either primary or secondary origin. Silver chloride has been noted 
in the cross-laminated Painted Desert sandstone in southwestern Utah. In 
western Colorado and southeastern Utah uranium and vanadium minerals 



1 Kay, Marshall, Analysis of Stratigraphy: Am. Assoc. Petroleum Geologists Bull., vol. 31, no. 1, 
pp. 161-181, Jan. 1947. 

^ Krumbein, W. C, Lithojacies Maps and Regional Sedimentary -Stratigraphic Analysis: Am. Assoc. 
Petroleum Geologists Bull., vol. 32, pp. 1909-23, 1948. 

^Dapples, E. C, Krumbein, W. C, and Sloss, L. L., Tectonic Control of Lithologic Associations: 
Am. Assoc. Petroleum Geologists Bull., vol 32, pp. 1924-47, 1948. 

* Sloss, L. L., Krumbein, W. C, and Dapples, E. C, Integrated Fades Analysis: Geo]. Soc. America 
Mem. 39, pp. 91-123, 1949. 



Stratigraphic, Structural, and Correlation Considerations 13 

occur disseminated throughout Jurassic and Triassic sandstones. Possible 
minor sedimentary copper deposits are widely distributed in Texas, New 
Mexico, Colorado, Utah, and Arizona. Manganese minerals (sulphides, 
oxides, carbonates, and silicates) are widespread in sedimentary sections 
west of the Mississippi Valley. Rich deposits of phosphate occur in cer- 
tain Permian strata in Idaho and Wyoming, in the Silurian and Devonian 
of Tennessee, and in the Tertiary of the Carolinas and Florida. Vast re- 
serves of potash (polyhalite and sylvite) are associated with Permian 
strata in New Mexico and west Texas. Other stratigraphically controlled 
deposits of nonmetallics include clay, borates, sulphur, gypsum, magnesite, 
barite, celestite, strontianite, diatomite, limestone, dolomite, slate, and 
marble. Lead and zinc deposits occur in limestone, dolomite, and calcar- 
eous shale in various parts of the world. The distribution of the Clinton 
iron ores (Silurian) of the Appalachian States and the hematite deposits 
of the Lake Superior region are governed mainly by stratigraphic fabric. 

The distribution, degree of localization, and value of many pyro- 
metasomatic ore deposits are largely dependent upon the type of sedimen- 
tary section intruded. Rarely are these deposits found in argillaceous 
strata, sandstones, and shales, whereas limestones, dolomites, and calcar- 
eous shales are more reactive and thus are most adaptable for mineral 
concentrations. 

Many deposits formed under mesothermal conditions (mineralization 
at 200°— 300° C.) occur in sedimentary rocks. Examples of mesothermal 
replacement deposits involving sedimentary strata are known in the Cordil- 
leran region of the United States and elsewhere in the world. 

Hypothermal or deep-seated (mineralization at 300°— 500° C.) ore 
accumulations are commonly associated with highly metamorphosed sedi- 
ments. 

Unconformities and variations in porosity, permeability, competency, 
composition planes, texture, and chemical composition of the host rock 
are some of the controlling factors governing the development and loca- 
tion of ore bodies. 

From the foregoing it is obvious that the stratigraphy of a mineral- 
ized area should be carefully evaluated during prospecting and develop- 
ment stages. The structural aspects of a region are equally important. 
Structural irregularities cannot be satisfactorily evaluated without knowl- 
edge of stratigraphic relationships. Similarly, stratigraphic values may 
be erroneously recorded if structural conditions are inadequately known. 

Subdivisions of Stratigraphic Geology 

Stratigraphic geology may be subdivided into two major categories, 
macrostratigraphy and microstratigraphy. The former involves field ob- 
servation and interpretation of exposed stratigraphic sequences, whereas 
the latter implies laboratory approach and routine and detailed evaluation 



14 Subsurface Geologic Methods 

of stratal successions. To obtain the maximum value from stratigraphic 
investigations both informational sources must be harmoniously synchron- 
ized. Field and laboratory personnel attacking a stratigraphic problem 
should be thoroughly aware of their limitations. 

The macrostratigrapher operates under diverse conditions, depending 
on the nature of the assignment (detailed, semidetailed, or reconnaissance) , 
the character and desired quality of results, the quality of assisting per- 
sonnel, the time allotted to the problem, and the field environment. He 
should be aware that inadequate field control promotes erroneous micro- 
stratigraphic conclusions, which may introduce unnecessary and excessive 
expenditure for the operating company. The macrostratigrapher is re- 
sponsible for field mapping, the interpretation of structural anomalies, 
the selection and definition of type outcrop sections, the collection of 
stratigraphically controlled representative samples, the orientation of facies 
variations, and the establishment of formational units. It is essential that 
the macrostratgrapher periodically became acquainted with problems 
confronting the microstratigrapher. 

The microstratigrapher controls and coordinates laboratory proced- 
ures essential for stratigraphic refinements. He must be versed in basic 
geologic and stratigraphic principles, their applications, limitations, and 
interrelationships, as well as be thoroughly familiar with methods required 
to decipher stratigraphic problems. The laboratory should be systemat- 
ically organized and the personnel suflSciently trained to minimize the time 
factor, as the macrostratigrapher is invariably concerned with knowing the 
results of the analyses of his samples as soon as possible. The microstrati- 
grapher should have knowledge of Foraminifera, ostracodes, diatoms, 
Radiolaria, and Mollusca, from which depositional-environmental deduc- 
tions may be made. The basic fundamentals of lithology, detrital min- 
eralogy, thin and polished sections, stain tests, insoluble-residue tech- 
niques, and porosity and permeability tests should be ably and efficiently 
applied whenever the occasion demands. The microstratigrapher should 
be aware of the principles and significance of electric, radioactive, thermal, 
caliper, and drill-time logging as these methods have either a direct or 
indirect bearing on the interpretation, evaluation, and correlation of 
stratigraphic sequences. 

Nature and Classes of Stratigraphic Units 

In 1933 a stratigraphic code, commonly cited as the "Ashley et al. 
report,"^ was published for the purpose of minimizing inconsistencies 
in stratigraphic terminology. For thirteen years this code served as a 
basis for stratigraphic standardization. In 1946 representatives of the 
Association of American State Geologists, the American Association of 



^Ashley, G. H., et al.. Classification and Nomenclature of Rock Units: Geol. Soc. America Bull., 
vol. 44, pp. 423-459, 1933; Am. Assoc. Petroleum Geologists Bull., vol. 17, pp. 843-863, 1933; vol. 23, 
pp. 1068-1069, 1939. 



Stratigraphic, Structural, and Correlation Considerations 15 

Petroleum Geologists, the Geological Survey of Canada, the Geological 
Society of America, and the United States Geological Survey met at Chi- 
cago under the chairmanship of R. C. Moore to discuss reorganization 
and improvement of the Ashley report. As a result of this meeting the 
American Commission on Stratigraphic Nomenclature was founded. The 
purposes of the commission 

. . . are to develop a statement of stratigraphic principles, to recommend 
procedures applicable to classification and nomenclature of stratigraphic units, 
to review problems in classifying and naming stratigraphic units, and to for- 
mulate expression of judgment.^ 

Distinction between time, time-rock, and rock units must be recognized 
by geologists before satisfactory stratigraphic concepts can be harmon- 
iously discussed. Renz '^ ably clarifies this necessity by saying : 

During the last decade, a number of geologists and stratigraphers in the 
United States have strongly advocated adopting uniformity in stratigraphic 
nomenclature and following more closely the original definitions of the terms 
to be used. Incorrect application of terms in stratigraphic geology causes con- 
fusion and misunderstanding, thereby impeding or even nullifying a clear con- 
ception of the stratigraphic conditions of areas to be studied from available 
publications. ... In setting up the stratigraphy of a given area, a clear dis- 
tinction has to be made between the classification of rocks into lithogenetic 
units of various magnitudes, such as groups, formations, and members, and the 
classification of the same rock sequences into time-stratigraphic units delimited 
by the vertical ranges of fossil life; such time-stratigraphic units are termed 
"series," "stages," "zones," etc. The corresponding time units, such as epoch, 
age, and secule (moment), express the interval of time during which these 
stratigraphic units were deposited. 

The establishment of lithogenetic units is the domain of the field geologist 
who maps them in the field according to the physical expression of the rocks 
only, without special reference to the stratigraphic range of the fossils they 
may contain. The paleontologist and biostratigrapher, on the other hand, build 
up their classification into time-stratigraphic units by studying the vertical 
range of fossil life. The classifications arrived at independently by the field 
geologist and by the biostratigrapher may, but often do not, coincide. In gen- 
eral, lithogenetic units have a rather limited geographic extent and are useful 
for correlation over comparatively small areas only. On the other hand, time- 
stratigraphic units are prone to exceed the geographic extent of lithologic 
units and, therefore, are more useful for regional or even interregional cor- 
relations. 

To promote better understanding of these stratigraphic terms, the 
American Commission on Stratigraphic Nomenclature has recommended 
the "following three classes of stratigraphic units: (1) "time units" for 
divisions of geologic time, (2) "time-rock units" for divisions of rocks 
segregated on the basis of their relation to determined segments of geologic 
time, and (3) "rock units" for divisions of rocks segregated on the basis 



^Organization, "nd Objectives of the Stratigraphic Commission: Am. Assoc. Petroleum Geologists 
Bull., vol. 31, no. 3, pp. 513-518, Mar. 1947. Prepared by R. C. Moore. 

' Renz, H. H., Stratigraphy and Fauna of the Agua Salada Group, State of Falcon, Venezuela: 
Geol. Soc. America Mem. 32, pp. 1-219, 1948. 



Stratigraphic, Structural, and Correlation Considerations 17 

of objective characteristics deemed to have significance in classification, a 
differentiation not based on time relations. (See figs. 5 and 6.) 

Time units, involving eras, periods, epochs, and ages, are defined 
indirectly and somewhat indefinitely on the basis of time-rock units. They 
represent time spans. 

Time-rock (time-stratigraphic) units, including systems, series, and 
stages, have time boundaries only and are represented by sediments de- 
posited during time intervals. Stratal thickness is not involved. The 
boundaries of time-rock units are essentially established on the basis of 
paleontology. Hedberg ^ comments: 

Fossils, of course, constitute one of the best means of both correlating and 
dating rocks; because of the more or less orderly evolutionary sequence of 
life forms on the earth (worked out, however, only through relation of fossil 
occurrences to the succession and superposition of strata), they constitute by 
far the most effective means of setting up a chronological system of time- 
stratigraphic divisions. However, there are limits to the resolving power of 
fossils as chronological indicators. While sediments differing in age by twenty 
million years, for example, may be readily placed in their correct sequence by 
an experienced paleontologist, smaller differences in time become progressively 
more difficult to place correctly, and a limit is finally reached where facies 
variations and otlier factors completely mask the changes in fossil record due 
to difference in age. 

Numerous other features besides order of superposition and paleontology 
can contribute evidence of relative age. Among these are radioactive measure- 
ments, relations to diastrophic events, evidences of volcanic activity, climatic 
changes, unconformities, sedimentary cycles, transgressions, and regressions. 
Many of these may, in special cases, become of outstanding importance and 
exceed in value all other means. However, only fossils (and perhaps radio- 
active measurements) are of much service in determining complete and world- 
wide geochronological sequences. . . . In short, it is desirable to be able to ex- 
press as a time-stratigraphic unit the sediments equivalent in age to the time 
scope of any recognizable features of sedimentary rocks which may be useful 
as a stratigraphic measuring stick. 

Relationships between time-rock and rock boundaries are difficult to 
establish, and many are impossible to evaluate accurately. 

Time surfaces may be defined by (1) careful study of stratigraphic 
sections containing lithologies and faunas common to two or more con- 
trolled stratal sequences; (2) "walking out" of key beds such as ash, ben- 
tonite, and limestone; (3) correlation of benthonic faunas possessing wide 
ecologic valence; (4) application of pelagic faunas and floras; (5) widely 
dispersed detrital minerals; (6) vertical limits of faunal sequences; and 
(7) biologic evolutionary changes. 

Rock units ( lithogenetic units), including the group, formation, 
member, lentil, tongue, stratum, and layer, are defined on the basis 
expressing structural conditions, and deciphering the geologic history of 
of lithology. These units are essential in geologic mapping, description, 

* Hedberg, H. D., Time-Stratigraphic Classification of Sedimentary Rocks: Geol. Soc. America Bull., 
vol. 59, pp. 447-462, May 1948. 



Stratigraphic, Structural, and Correlation Considerations 



19 



expressing structural conditions, and deciphering the geologic history of 
a region. Rock units may be objectively shown on a map and in strati- 
graphic sections. Their boundaries do not necessarily coincide with time 
boundaries. Several rock units may be incorporated with a time-rock unit. 
It has been indicated ^ by the American Commission on Stratigraphic 
Nomenclature that in treating a sedimentary formation (rock unit), the 
following points should be considered: (1) It must contain no apparent 
evidence of an appreciable break in deposition. (2) In its simplest form 
it consists dominantly of one general type of rock. (3) Recognition of 
the same formation in different areas is only justified when its essential 
lithologic definition is applicable. (4) The upper or lower contacts of a 
sedimentary formation may laterally transgress horizons of a neighboring 
formation. (5) The top and bottom of a sedimentary formation are de- 
fined either by a change in lithology or by evidence of an appreciable 




Figure 7. Time-space relationships of sedimentary deposits. Tr and Ti. represent time 
surfaces. Changes in sedimentary rock types must always be considered in three 
dimensions. 



interval of nondeposition. (6) A formation may hold one or more faunas 
or floras. (7) A sedimentary formation may include minor developments 
of volcanic rocks. (8) Pyroclastic materials, whether deposited in water 
or on land, are to be regarded as volcanic sediments and, hence, as con- 
stituents of sedimentary formations. 

In naming surface rock units the following points should be con- 

^ Rules of Geologic Nomenclature of the Geological Survey of Canada: Am. Assoc. Petroleum Geol- 
ogists Bull., vol. 32, no. 3, pp. 366-381, Mar. 1948. Prepared by R. C. Moore. 




CD 


•|-i 


U 






n 






O. 


U) 


-a 




d 


U 


o 












cd 


S 


(I 





S o 



J3 



J3 a^ 

■^i TJ en 
C en 

o £ 

OJ ij tn 






•J2 " -Q 
2 »-o 

o ^ > 

c ^ 

o ^^ tn 
ID 

<^.-^ a 

(D -2 

tn • j3 

^^ u 
=0.2 '5 
« S o 



fe 



Stratigraphic, Structural, and Correlation Considerations 21 

sidered: (1) the description of the type locality or area; (2) the detailed 
and summary lithologic description; (3) a statement pertaining to thick- 
ness variations; (4) a statement concerning the surface and subsurface 
distribution of the unit; (5) a discussion of stratigraphic relationships; 
(6) comments on detrital mineralogy and paleontology; (7) a statement 
pertaining to physiographic expression; (8) a discussion of correlation; 
(9) comments on environment of deposition; and (10) economic aspects. 
Similar procedure should be followed in defining groups, members, lentils, 
or other rock units. 

In naming subsurface rock units the American Commission on Strati- 
graphic Nomenclature ^° recommends that : 

Subsurface units may be given formal names when such names are neces- 
sary for adequate presentation of the geologic history of the region or when 
the subsurface section is materially different from equivalent exposed beds. 

(a) Subsurface units, recognized and named from Avell logs, including 
sample logs, core records, and electrical and other mechanically recorded logs, 
with the assistance of paleontological determinations differ from surface units 
in that the "type locality" cannot be visited and restudied from time to time 
by subsequent workers. With the advent and wide dissemination of the elec- 
trical log and the systematic saving of representative well cuttings, particularly 
in areas of deep drilling, it frequently has become possible, in recent years, to 
bring the "type locality" into the office or laboratory of any truly interested 
worker. In fact, the log portion of the "type locality" can and should be pub- 
lished. This condition lessens the need for restricting the number of names 
of subsurface units to an irreducible minimum. If a subsurface type section is 
definitely better and more typical than that available at any surface type local- 
ity, the subsurface section may be designated as type. Otherwise, a surface 
type locality shall be designated. 

(6) Names applied to subsurface units shall be governed by the same 
restrictions and regulations as prevail for exposed units. [See Articles 7, 9, 10, 
and 11 of the original paper.] 

(c) When it becomes possible to correlate a named subsurface unit with 
a named surface unit, and when the surface and subsurface facies are suffi- 
ciently similar that two names are unnecessary, the name of the surface unit 
is to be applied, even though the subsurface name has priority, unless much 
more extensive usage of the subsurface name renders its retention preferable or 
necessary. 

(d) When beds are discovered which are equivalent and in similar facies 
to a named subsurface unit, the name of the subsurface unit shall have priority. 

(e) When it is found that a subsurface unit has been named for but mis- 
correlated with a named surface unit, a new name shall be given the subsurface 
unit or it shall be renamed for its- true correlative on the surface. In rare 
instances, exceptionally widespread use of the name by the subsurface unit 
may make it advisable to permit the "pirating" of the name by that unit and 
thus force the renaming of the surface unit from which it derived its name. 
Such "pirating" should be held to a minimum and should be accepted for pub- 
lication only after a favorable ruling by the American Commission on Strati- 
graphic Nomenclature. 



^^ Naming of Subsurface Stratigraphic Units: Am. Assoc. Petroleum Geologists Bull., vol. 32, no. 3, 
pp. 369-370, Mar. 1948. Prepared by W. V. Jones and R. C. Moore. 



22 Subsurface Geologic Methods 

(/) In proposing a new name for a subsurface unit, it is desirable to de- 
scribe for the type section the following features: 

(1) Location of the type locality well; name of operating company or 
individual; date of drilling; results and present status of the well; elevation 
of surface at the well and depths to top and bottom of the new unit. 

(2) If all data needed to establish the type section properly cannot be 
furnished from one well, two or more wells shall be used and the data called 
for under (1) shall be furnished for each well so used. 

(3) As complete a section as possible shall be described in detail from 
cores of the new unit. Where sample logs are available, critical portions of 
them shall be included in written or graphic form or both. The boundaries 
and subdivisions, if any, of the new unit shall be indicated clearly in these logs 
and core records. 

(4) Where electrical or other mechanically recorded logs are available, 
the critical parts of such logs, preferably of several wells located in a single 
area, shall be published in the article proposing the new named unit. The 
boundaries and subdivisions, if any, of the new unit shall be marked plainly 
on these published logs, which shall be on a scale large enough to permit full 
appreciation of all details of the new unit. 

(5) Fauna and flora. Diagnostic fossils of the new unit shall be described 
in detail and, if possible, figured. Description and figuring of diagnostic fossils 
are essential if the new unit is a time-rock unit. 

(6) Nature of underlying and overlying units. 

(7) Correlation and position in the general stratigraphic scale. 

(8) Present location of the cuttings or samples. 

(9) Present location of the fossils. 

(10) Critical parts of written driller's logs of all wells used, unless these 
are considered to be so inaccurate that their inclusion would be confusing. 

(g) The cuttings and the fossils, accompanied by copies of all available 
types of logs should be placed in some official, permanent depository. As a 
rule, the appropriate state geological survey will serve as such a depository. 

(h) The editorial staffs of all publishing agencies are urged to insist that 
the provisions of this article be followed in detail whenever a subsurface unit is 
being given a name. 

Facies Concept 

Although the principle of facies and facies changes has long been 
recognized by stratigraphers, it has only been during the past ten years 
that more serious consideration has been given the concept. Those in- 
terested in sedimentary facies are referred to the following: "Intertongu- 
ing Upper Cretaceous Deposits" by W. S. Pike, Jr., (Geol. Soc. America 
Mem. 24, 1947), "Sedimentary Facies in Geologic History" (Symposium) 
(Geol. Soc. America Mem. 39, 1949) and "Sedimentary Facies in Gulf 
Coast" by S. W. Lowman (Am. Assoc. Petroleum Geologists Bull., vol. 33, 
no. 12, pp. 1939-1997, Dec. 1949). 

Neglect of the facies concept in stratigraphic geology has led to many 
questionable and ill-founded correlations. The recognition and evaluation 
of facies changes are cardinal to the proper establishment of the strati- 
graphic and structural fabric of any area. 

The term "facies" has been variously interpreted by stratigraphers, 



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24 Subsurface Geologic Methods 

and, as a result, considerable confusion prevails in its use. According to 
Moore,^^ "sedimentary facies are areally segregated parts of differing 
nature belonging to any genetically related body of sedimentary deposits." 
He also states that the term "lithofacies" "denotes the collective characters 
of any sedimentary rock which furnish record of its depositional environ- 
ment." A sedimentary facies may involve a member or stratum, a forma- 
tion or group, or a time-rock unit. 

The term "facies" has been used by some workers without reference 
to stratigraphic units; for example, "red bed facies," "reef facies," "la- 
goonal facies," "marine facies," and "evaporite facies." As regards this 
usage, Moore comments, "There is value in this sort of classification of 
sedimentary deposits and in comparing rocks presumed to have been formed 
under like environments, without regard to age or local geologic settings." 

To prove the time equivalency of dissimilar facies is exceedingly 
difl&cult, particularly when data are limited. The solution of this problem 
lies in (1) establishing regional stratigraphic trends; (2) carefully plan- 
ning areal mapping programs; and (3) detailing and correlating control 
stratigraphic sections. 

Sloss ^^ et al in an excellent discussion on the facies problem have 
suggested four methods of approach in analyzing sedimentary facies: (1) a 
paleogeographic approach, wherein a study of facies and their distribution 
patterns in time and space attempts a reconstruction of ancient source 
areas and depositional environments and their distribution in past geo- 
graphic patterns; (2) a biologic approach, which is based on the recon- 
struction of paleoecology from the study of the biologic complex occurring 
in fossiliferous strata; (3) an oceanographic approach, which involves the 
collection and integration of environmental data governing recent sedi- 
mentation, which in turn may be helpful in interpreting ancient deposits; 
and (4) a tectonic approach, which is based on the study of the tectonic 
behavior of any area and the facies response to such behavior. 

Facies changes vary in type and magnitude throughout the geologic 
column. These changes may be pronounced in relatively short distances, 
others may be gradual over extended distances. Several examples of facies 
variations may be cited : the Middle Tertiary of central and south Sumatra, 
the Upper Cretaceous of northern and central Egypt, the Upper Cretaceous 
of the Rocky Mountain region, the Permian of Russia and of the Permian 
Basin of Texas and New Mexico, the Tertiary of the Gulf Coast and 
Pacific Coast of the United States, and the Tertiary of northern South 
America. 

The Devonian of New York State presents an excellent example of shifting 
facies, particularly in the post-Onondaga parts where the changes of sediments 
have been traced from red beds in eastern New York to black shales and lime- 



" Moore, R. C, Meaning of Facies, Geol. Soc. America Mem. 39, pp. 1-34, 1949. 

^^ Sloss, L. L., Krumbein, W. C, and Dapples, E. C, Integrated Facies Analysis, Geol. Soc. America 
Mem. 39, pp. 91-123, 1949. 



Stratigraphic, Structural, and Correlation Considerations 25 

stones in Ohio. In general, the change is from red sands and conglomerates 
to gray, fine-grained sandstones to dark siltstones, thin to dark-gray and black 
shales, and finally to calcareous shales and limestones containing coral planta- 
tions and bioherms. The pattern of these shifts is now so well known that 
relationships not yet recognized in parts of the Appalachian geosyncline can 
be anticipated.^^ 

According to Weller:^^ 

Facies variation is not peculiar to the Mississippian system [North Amer- 
ica], but the problems which result from certain types of rapid lateral variation 
in sediments, in the fields of both practical stratigraphy and stratigraphic 
nomenclature, have received more attention in the Lower Mississippian and 
Upper Devonian rocks of Ohio, Pennsylvania, Indiana, and Kentucky than in 
other parts of the stratigraphic column and other regions of the continent. 

The work of Hyde,^^ Stockdale,!^ ^'^ Chadwick,!^ and Caster ^^ is partic- 
ularly important. Both Hyde and Stockdale recognized certain major forma- 
tions, each of which, they presumed, was deposited contemporaneously through- 
out its extent. Each formation was then divided vertically into different 
facies developments, which were given geographic names. Finally the facies 
were divided horizontally into members which were also partially or completely 
named. This system has proved to be flexible and convenient for description. 
The facies names are, in effect, synonyms of the formation names but have 
only local significance. Although this system introduces a large number of 
names, many of them can be ignored by persons having no interest in the 
detailed stratigraphy of these formations. 

Caster was more concerned with the interrelationships of rock-stratigraphic 
and time-stratigraphic units. To units of more or less uniform lithologic char- 
acters which transgress time lines he applied the term "magnafacies"; these are 
rock-stratigraphic units and correspond to the original lithologic formations 
of the northern Appalachian region. He used the terms "stage," "formation," or 
"stratigraphic unit" for time-stratigraphic units. The magnafacies and stages 
intersect each other, and for the intersected strata Caster introduced the term 
"parvafacies." According to this system, which is an elaboration of conclusions 
reached earlier by Chadwick and others, each magnafacies consists of a suc- 
cession of parvafacies of similar lithologic character but unequal age, and each 
stage consists of a succession of parvafacies of similar age but different litho- 
logic character. Geographic names were introduced for all so that strata at any 
place have three major names: a more or less local parvafacies name, extensive 
magnafacies, and stage names. In addition, named members are also recog- 
nized. This system is of considerable theoretical interest and may be very 
useful in the detailed study and description of the strata deposited near the 
margin of an expanding delta. It is not likely, however, to be widely applicable 
to other types of facies problems. 



^^ Cooper, A. G., et al., Conelation of the Devonian Sedimentary Formations of North America: Ceol. 
Soc. America Bull., vol. 53, no. 12, pp. 1729-1794, 19<12. 

^'' Weller, J. M., et al.. Correlation of the Mississippian Formations of North America: Geol. Soc. 
America Bull., vol. 59, uo. 2, pp. 91-196, 1948. 

■•^ Hyde, J. E., Stratigraphy of the Waverly Formations of Central and Southern Ohio: Jour. Geology, 
vol. 23, pp. 655-682, 757-779, 1915. 

^° Stockdale, P. B. , The Borden (Knobstone) Rocks of Southern Indiana: Indiana Dept. Cons. 
Pub. 98, 1931. 

'' Stockdale, P. B,, Lower Mississippian Rocks of the East-Central Interior: Geol. Soc. America 
Special Paper 22, 1939. 

^^ Chadwick, G. H., Faunal Differentiation in the Upper Devonian: Geol. Soc. America Bull., vol. 
46, pp. 305-342, 1935. 

Caster, K. E., The Stratigraphy and Paleontology of Northwestern Pennsylvania, pt. 1: Am. 
Paleontology Bull., vol. 21, no. 71, 1934. 




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Stratigraphic, Structural, and Correlation Considerations 27 

In a classic paper on the sedimentary facies of the Gulf Coast, Low- 
man "^^ says : 

As the search for oil turns more and more toward a search for new prov- 
inces and new trends, we realize the inadequacy of present methods of eval- 
uating the significance of sedimentary properties and their patterns of distri- 
bution. . . . The problem of discovering sedimentary criteria for the recogni- 
tion of petroleum provinces and trends surely involves the study of sedimenta- 
tion, stratigraphy, and structural history. 

Lowman emphasizes that the logical place to begin a fundamental 
investigation of facies would appear to be in the environments of deposi- 
tion and states: 

In a search for sedimentary criteria by which we may identify environ- 
ment of deposition of sedimentary rocks, it may be useful to classify the sedi- 
mentary variables as "static" and "dynamic." Among the static variables may 
be listed those properties which retain their original depositional characteristics 
(although they may be somewhat modified by postdepositional processes) as 
follows: (1) gross mineral composition of the solid rock, for example, lime- 
stone, shale, or sandstone; (2) size, shape, and distribution of detrital grains; 
(3) fossils; (4) sedimentary structures; and (5) possibly some mass effects 
related to chemical composition, such as radioactivity and magnetic properties. 
Among the dynamic variables might be included: (1) minor changes in gross 
composition, for example, limestone to dolomite; (2) cement; (3) chemical 
composition of rock fluid; (4) character of those parts of the mineral assem- 
blage which are susceptible to postdepositional change through differential 
loss of some minerals and authigenic gain of others; and (5) some physical 
mass properties such as density and porosity. Stated in another way, the static 
variables are those which should be useful in making interpretations of deposi- 
tional environments. The dynamic class, on the other hand, should serve in 
making interpretations of post-depositional change. 

Dapples et al -^ express a fundamental concept that cannot be min- 
imized in facies interpretation. It is believed by these writers that: 

The tectonic behavior of the depositional area is the most important fac- 
tor in the control of lithofacies, and the environment of deposition (littoral, 
nertic, et cetera) plays a part which depends on the length of time environ- 
mental conditions can affect the material before it is buried. The source 
area, except in special instances, appears to be a less important factor. The 
tectonic behavior of the depositional area itself includes several factors, among 
which are the geographic distribution of tectonic elements, and the intensity 
of the tectonism in each. 

Pettij ohn ^^ comments that : 

The fundamental cause of the observed differences in lithology and associ- 
ated phenomena has been the rate of sedimentation which, in turn, is controlled 
by the related rates of elevation and depression of the source area and the 



^Lowman, S. W., Sedimentary Facies in Gulf Coast: Am. Assoc. Petroleum Geologists Bull., vol. 33, 
no. 12, pp. 1939-1997, Dec. 1949. 

"Dapples, E. C, Krumbein, W. C, and Sloss, L. L., Tectonic Control of Lithologic Associations: 
Am. Assoc. Petroleum Geologists Bull., vol. 32, no. 10, pp. 1924-47, 1948. 

^ Fettijohn, F. J., Sedimentary Rocks, p. 436, Harper's Geoscience Series, 1949. 



28 Subsurface Geologic Methods 

basin of sedimentation, respectively. Tectonics is indeed the soul of the matter. 
Therefore, the first breakdown of environments must be tectonics. 

The importance of facies development and interrelationships have too 
often been neglected and minimized in favor of the structural impress in 
selecting potential petroliferous areas. The definition and evaluation of 
facies changes are as important in outlining favorable petroliferous areas 
as are the structural anomalies developed within them. 

The question arises: What is the importance of evaluating facies 
changes? There are several answers: 

1. Lateral changes commonly involve variations in porosity and 
permeability, which control selective accumulation of liquid and gaseous 
hydrocarbons. 

2. Selection of areas containing proper facies and facies relation- 
ships may reduce exploratory costs for an oil company. The facies factor 
has many times been more important in controlling oil and gas accumula- 
tions than structural conditions. 

3. Paleogeologic data may be more accurately interpreted when 
lateral variations and relationships of the sediments are properly inte- 
grated. 

4. Before faunal and stratal sequences can be properly arranged 
chronologically, a knowledge of facies relationships must be known. 

5. Variations in rock types may possibly control localization of 
ore deposits. 

6. Migrating lithofacies boundaries may be incorrectly interpreted 
as structural reflections. 

7. Changes in facies may complicate engineering problems, such 
as tunneling, excavating, and estimating costs. 

Facies changes result from fluctuation of sea level, climatic varia- 
tions, modification of topographies, diastrophic readjustments of the hin- 
terland, changes in oceanic currents, and drainage patterns, erosional 
cycles, readjustment in deposition basins, and migration of shore lines. 

When one is introduced into an unfamiliar province the approach to 
the solution of stratigraphic problems is first to generalize the stratigraphy 
and structure of the area and second to give consideration to the details of 
the section. This stage is followed by repeated regeneralization and re- 
detailing. Overemphasis of either generalizing or detailing may retard 
progress or promote erroneous interpretations. 

Reliable stratigraphic information demands accurate field control. 
Without adequate structural information, stratigraphic sequences and 
facies variations of deposits cannot be properly allocated. An unrecognized 
fault or nonrecognition of dissimilar facies equivalents may increase or 
decrease the normal thickness of stratal sequence by thousands of feet. 
The failure to recognize unconformities may introduce conflicting strati- 
graphic interpretations. 



30 Subsurface Geologic Methods 

Examples of Modern Fades Changes 

Revelle and Shepard ^^ show in figure 11 the general distribution of 
sediments and rock bottom off the coast of southern California. The types 
of these recent sediments fall into four categories: (1) sand, (2) sand and 
mud, (3) mud, and (4) calcareous. The distribution of these deposits is 
to a considerable extent controlled by submarine topography, "The ridges 
and saddles, whatever their depth and distance from shore, have notably 
coarser sediments than the depression and troughs." (See fig. 11) The 
distribution of the calcareous deposits in the area are extremely variable. 

Tracey -^ et al in their studies of the Bikini reef discuss the distribu- 
tion of corals and algae of the more important types and the relationship 
of channels, caverns, pools, and detrital deposits. They recognize a number 
of distinct facies zones roughly parallel to the reef front. It is stated 
that "Differences in the composition of the reef surface and in organic 
growth are also observable laterally — along lines parallel to the reef 
front — but these differences are less striking than the banding." Specific 
facies of the reef include (1) a marginal zone on the windward side (Litho- 
thamnium abundant), (2) a coral-algal zone (inside the marginal zone), 
(3) a reef flat (forms the major part of the reef and consists of eroded 
coral and algae with some foraminiferal sand), (4) a beach, and (5) 
lagoonal. 

As ancient reefs in carbonate sections are now being seriously con- 
sidered by oil companies in their exploration programs, modern reef 
development and distribution should be carefully and systematically 
analyzed. 

Lowman ^^ in a worthy contribution has discussed the modern facies 
of the Gulf Coast region. 

Examples of Ancient Facies Changes 

Many examples of ancient facies changes may be cited. One of the 
classic examples of lateral variations in strata is that reported on by 
King -^ within the Permian of the Guadalupe and Glass Mountains (fig. 
10) , of west Texas and New Mexico. Three well-defined facies are recog- 
nized in the Guadalupe series: (1) shelf (back-reef), (2) marginal (reef), 
and (3) basinal. The shelf facies is represented by limestone, evaporites, 
and minor amounts of sandstone. The marginal facies is dominated by 
the Capitan reef limestone. Sandstones, shales, and occasional thin lime- 
stones comprise the basinal facies. 



^Revelle, R., and Shepard, F. P., Sediments of} the California Coast: Recent Marine Sediments, 
a Symposium: Am. Assoc. Petroleum Geologists, p. 245-281, 1939. 

^^ Tracey, J. I., Ladd, H. S., and Hoffmeister, J. E., Reefs of Bikini, Marshall Islands: Geol. Soc. 
America Bull., vol. 59, pp. 861-878, 1948. 

^Lowman, S. W., Sedimentary Facies in. Gulf Coast: Am. Assoc. Petroleum Geologists Bull., vol. 
33, no. 12, pp. 1939-96, Dec. 1949. 

^' King, P. B., Permian of West Texas and Southeastern New Mexico: Am. Assoc. Petroleum Geolo- 
gtsts Bull., vol. 26, no. 4, pp. 535-763, 1942. 



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32 Subsurface Geologic Methods 

Adams ^^ admirably discusses the facies changes within the upper- 
most Permian deposits of southwestern United States. 

Dunbar -^ reports that the Kazanian series about Perm (Russia) 

is represented by a bright-red, unfossiliferous sandstone and siltstone some 500 
feet thick. This is the nonmarine Ufimian facies, so called for the city of 
Ufa, which is on the strike south of Perm. In conspicuous contrast, the equiv- 
alent beds in the environs of Kazan are richly fossiliferous white limestone. 

The Upper Cretaceous of the Rocky Mountain region of Colorado and 
Wyoming offers excellent examples of facies changes wherein the section 
becomes more arenaceous and less marine from east to west. 

Improving Interpretation of Stratigraphic Sequences 

Confusion and disagreement in stratigraphic studies often arise as a 
result of one or more of the following: (1) projection of stratigraphic 
nomenclature of one area into another without sufficient intervening con- 
trol; (2) failure to distinguish between rock and rock-time units; (3) 
publication of stratigraphic material by those who are inexperienced or 
who have drawn conclusions based on inadequate data; and (4) failure 
to recognize the importance of structural and depositional idiosyncrasies. 

The evaluation of stratigraphic sequences has been greatly improved 
by application and coordination of the following studies: (1) detailing 
of surface and subsurface sections; (2) subsurface logging techniques; 
(3) detailed paleontologic and mineralogic analysis; (4) improved lab- 
oratory and field procedures; and (5) better understanding of sedi- 
mentational principles. 

Projects devoted to the study of clay minerals, bacteria, organic and 
carbonate sediments, formation waters, recent and ancient sedimentary 
processes, diagenesis, and well logging, as now being sponsored by the 
American Association of Petroleum Geologists, will contribute greatly 
in more accurately evaluating stratigraphic sequences. 

Unconformities 
W. ALAN STEWART 

Unconformities are of prime importance in problems of strati- 
graphic, sedimentational, structural, and historic nature. They are the 
natural basis for separating both rock units and units of geologic time. 
The economic importance of unconformities is constantly increasing be- 
cause of their relationship to oil accumulation and to ore deposition. 
The occurrence, development, extent, type, and relationships of uncon- 
formities are therefore of major concern in petroleum and mining de- 
velopment programs. 

Since the detection of unconformities in the subsurface is of para- 
mount value to the stratigrapher and economic geologist, much emphasis 



^' Adams, J. E., Upper Permian Ochoa Series of Delaware Basin, West Texas and Southeastern New 
Mexico: Am. Assoc. Petroleum Geologists Bull., vol. 28, no. 11, pp. 1596-1625, Nov. 1944. 

^"Dunbar, C. O., Permian faunas: A Study In Facies: Gaol. Soc. America BulL, vol. 52, p. 313-32, 
1941. 



Stratigraphic, Structural, and Correlation Considerations 33 

is given to the criteria for unconformity recognition. Considerable space 
has been devoted to the use of unconformities in interpreting the geologic 
record. A number of examples of unconformity-type oil fields are de- 
scribed and illustrated. A section on the influence of unconformities on 
ore deposits is also included. 

An unconformity has been defined by Twenhofel ^^ as a surface of 
erosion or nondeposition separating two groups of strata. Unconform- 
ities that are surfaces of erosion are more common than surfaces of non- 
deposition, possibly because they are more easily recognized. Surfaces 
of nondeposition indicate a long time break in sedimentation and may 
leave little evidence of their occurrence in the geologic column. Such a 
break would involve the establishment of a profile of depositional equi- 
librium for a long period of time. 

The time value of an unconformity is the interval of geologic time 
represented by the break. This ranges from the time required to deposit 
a single formation to the enormous time interval represented by the un- 
conformity between the pre-Cambrian and Pleistocene glacial deposits. 
Twenhofel ^^ aptly likens all unconformities to the branches of a tree, 
the trunk of which is the present land surface and whose branches spread 
out to intercalate in all directions through the strata. Thus every un- 
conformity directly or through another unconformity intersects the pres- 
ent land surface. 

The relief on surfaces of unconformity may vary from peneplanation 
to several thousand feet. The relief on the Ep-Algonkian surface of the 
Grand Canyon is at least 800 feet, while Carboniferous sedimentary rocks 
around Boston, Massachusetts, rests unconformably on a surface of at 
least 2,100 feet. The amount of relief on an unconformity is no indication 
of the time value of the unconformity. A surface of great relief might 
represent only a small fraction of the time involved in the formation of a 
peneplained surface of slight relief. Conversely, a surface of small relief 
may not have passed through the initial stages of subaerial or submarine 
erosion, and may involve only a short period of geologic time. The phys- 
ical appearance of an unconformity should not be used to gage its time 
value. Fossil evidence, if present on both sides of an unconformity, is the 
most reliable criterion of time value. 

Nomenclature 

The various kinds of unconformities depend for their classification 
upon the attitude of the strata on both sides of the unconformity, the 
genesis of the rocks involved, and their areal extent. 

Disconformity. In a disconformity, the strata on both sides of the 
unconformity are parallel. The older strata have lain undisturbed in 
position during the break in deposition. The younger beds have then 

^' Twenhofel, W. H., Treatise on Sedimentation, Baltimore, Md., Williams and Wilkins Co. 1926. 
^'^Ibid., p. 447. 



34 



Subsurface Geologic Methods 



been deposited with their stratification parallel to that of the older beds. 
This type of unconformity has been called a parallel unconformity by 
Lahee,^^ and they have been aptly named stratigraphic unconformities 
by Grabau.^" 

The term "diastem" may be used to designate a disconformity whose 
time value is less than that of a formation. 

Disconformities may be difficult to detect, especially if there is little 
relief on the surface. Evidences of erosion, if present, may simplify the 
task. If no erosion is detected, then fossils are the best means of deter- 
mining disconformity. 

Nonconformity. In a nonconformity, the strata below the uncon- 
formable contact are at an angle with those above. This type may be de- 




FiGURE 13. Cores taken two feet apart from a well section in central Sumatra. 
Observe difference in dip; this difference may be attributed to variance in 
competency of lithologic types, to irregularity in deposition, and perhaps to 
preconsolidation slumping movement. If these two sections were tak n as spot 
cores several hundred feet apart stratigraphically a nonconformity could possibly 
be inferred. 



scriptively called an "angular unconformity." According to Grabau,^^ 
this is the true unconformity where there is a discordance of strata, and 
he has called them "structural unconformities" to contrast with his strat- 
igraphic unconformities. 



^^Lahee, F. H., Field Geology, 4th ed.. New York, McGraw-Hill Book Co., 1941. 

'^ Grabau, A. W., Guide to the Geology oj the Schoharie Valley in Eastern New York: New York 
State Mus. Bull. 92, 1906. 
»' Idem. 



Stratigraphic, Structural, and Correlation Considerations 



35 



The term "nonconformity" has been used extensively in the litera- 
ture to include unconformable contacts of sedimentary rocks with meta- 
morphic or plutonic rocks that have been subjected to erosion before 
deposition of sediments. Billings ^^ says that the term is utilized most 
satisfactorily for unconformities where the older rock is plutonic in or- 
igin, and uses "angular unconformity" to designate the unconformable 
contacts of discordant strata. Willis,^" however, suggests that the term 
be used in a broader sense to include any unconformity that is not a dis- 





FiGURE 14. Core section showing the effect of sliding movement in sediments during 
the hydroplastic stage. Structural interpretations cannot be depicted from data 
of this type. 

conformity. This is probably the most satisfactory application of the 
term. 

A nonconformity or angular unconformity involves a surface sep- 
arating two or more stratal units at an angle of discordance, which may 
vary from 90 degrees to less than 1 degree. If the latter variation exists, 
it is sometimes difficult to determine whether the discordance angle is 



' Billings, M. P., Structural Geology, p. 243, New York, Prentice-Hall, 1942. 

' Willis, Bailey and Robin, Geologic Structures, 3d ed., rev., New York, McGraw-Hill Book Co., 1934. 



36 Subsurface Geologic Methods 

attributable to deformation or to variance in initial depositional dip 
without extensive and accurate field mapping. It is even more difficult to 
make this determination if interpretations must be made on limited sub- 
surface evidence. (See figs. 13, 14.) 

Local Unconformity. A local unconformity is similar to a discon- 
formity, but differs in that it is of small areal extent and represents a 
comparatively short time interval. It is commonly the result of stream 
action in continental deposits. During times of flood, streams may scour 
out channels in their flood plains, which may be many feet wide and deep 
and thousands of feet to several miles in length. As the flood subsides, the 
channel may fill up, or it may fill up days or even years later. The strati- 
fication above such an unconformity is parallel to that below, making it 
a disconformity. The term "local unconformity" is used to indicate its 
small areal and short time value. 

An example of a local unconformity in a marine section would be 
the wave-truncated surfaces of some buried bioherms. These surfaces 
would develop on the seaward side of the reef, as the bioherms built up- 
ward and shoreward during a marine transgressive overlap. The bench 
cut by wave action would represent an erosion surface of limited ex- 
tent. If this surface was then buried by further advances of the sea, a 
local unconformity would result. 

The erosion involved in local unconformities is accomplished by a 
cessation of the upbuilding of the deposits and is called "contemporane- 
ous erosion." Local unconformities may be produced by contemporane- 
ous erosion in marine deposits by changes in strength or direction of 
marine currents; in lake deposits by similar changes in currents; and in 
eolian deposits by variations in wind. 

Blended Unconformity. A surface of erosion may be covered with a 
thick mantle of residual sands and gravels that grade downward into 
the rock from which they were derived. If they are later overlain by 
sediments, they may become the basal member of a new formation. In 
such a case, no distinct surface of separation can be seen, and the feature 
is a blended unconformity. 

Genesis of Unconformities 

Continental Unconformities. Continental unconformities are those 
whose surface is cut by a continental agent, succeeding deposits being of 
continental origin. The agent of deposition may be fluvial, eolian, or 
glacial. 

Aqueous: Fluvial unconformities are found in valley-flat, or in 
alluvial-fan or deltaic deposits. 

Unconformities in valley flats may arise from scour-and-fill and 
from the meandering of aggrading streams. In scour-and-fill, uncon- 
formities may be abundant and the relief is apt to be locally great, and 
correlation difficult or impossible. The relations are those of a discon- 



Stratigraphic, Structural, and Correlation Considerations 37 

formity, but as initial dips may be high, a pseudononconformable rela- 
tionship may exist. 

An aggrading stream building a flood plain of construction will oc- 
cupy at one time or another every position in the deposit. This is ac- 
complished through migration of meanders or by the stream breaking 
natural levees and seeking the lower land of the back swamps. As a 
flood plain often contains swamps and lakes, the unconformity developed 
may be between fluvial, paludal, or lacustrine deposits. Unconformities 
developed by aggrading streams have disconformable contacts and have 
a short to moderately long time value. 

Alluvial fans are deposited by stream distributaries on surfaces of 
varied origin, over which they extend outward from the uplands. The 
buried surface becomes an unconformity that may represent a long or 
short time value and may be either a disconformity or a nonconformity. 

Unconformities in deltas may be either continental or marine. A 
delta may advance or retreat according to the balance between supply 
and deposition. This advance and retreat leaves unconformable surfaces 
which may separate fluvial, paludal, lacustrine, or marine sediments, 

Eolian : The work of the wind may result in unconformable contacts 
of either type. Sand may be deposited by the wind on surfaces not pre- 
viously affected to produce unconformities on burial. Winds may erode 
earlier eolian deposits and later bury the eroded surface. Discordance 
between cross laminations may give such a contact the appearance of a 
nonconformity. The relief on eolian unconformities may be none to very 
great and the time value small to large. 

Glacial: The advance of a glacier will be accompanied by erosion, 
which tends to decrease the relief of the land surface over which it is 
passing. Deposition of morainal material on this surface may result in 
an unconformity of either type. 

Marine Unconformities. Marine unconformities are those found in 
marine deposits or between marine deposits above and deposits of another 
environment below. 

Overlap: Deposition of the basal sediments of a formation on an 
erosion surface are not likely to be laid down uniformly and contem- 
poraneously over a wide area. Actually, deposition will begin in a few 
favorable places and spread to other areas as formations become thicker. 
Thus succeeding strata will overlap the older rocks below the erosion 
surface. 

As the land is submerged by the rapid rise of an encroaching sea, a 
condition of overlap is created which has been called the "unconformity 
of progressive overlap" by Grabau.^^ The coarse debris of the land sur- 
face forms a basal conglomerate, and there may be buried soils. As de- 
posits are made in the advancing sea, each depth is characterized by 

'° Grabau, A. W., Principles of Stratigraphy, p. 723, 1913. 



38 



Subsurface Geologic Methods 



sediments related to depth. Thus a coast line will be characterized by 
roughly parallel belts of beach sands, muds, and limy oozes. As the sea 
moves in on the land, these three types of deposits will retain their rela- 
tive positions. Muds will be deposited on sands and oozes on muds. A 
vertical cross section through a marine transgressive overlap will show 
coarse elastics overlain by finer elastics. The relief on a surface of trans- 
gressive overlap is essentially that of the submerged surface. Figure 15 
shows a marine transgressive overlap. 

The slow rise of the sea may result in a different kind of progressive 
overlap. Erosion may take place before deposition and a wave-cut sur- 
face extending a long distance from shore may be developed. As the 
sea level continues to rise, the wave-cut surface is brought below the base 
level of erosion, and sediments are deposited. There will be fine-grained 



A 




F5^ 


























5fe 


5^cr 


^ 


^ 


^ 
^ 








^ 


^ 



Figure 15. A marine transgressive overlap; lines parallel to ocean floor are called 
"time lines" (ab) , for they run through material deposited contemporaneously; 
lines essentially parallel to gravel, sand, and mud deposits are called "formation 
lines" icd) . A-B — Erosion surface overlain unconformably by overlapping sedi- 
ments. 



sediments, and the basal conglomerate produced by rapid submergence 
will not be present. 

Unconformities resulting from overlap may be found in lacustrine 
and alluvial fan deposits as well as marine. 

Ofilap : Offlap or regressive overlap is produced when the sea recedes 
from the land. The beach zone will migrate over earlier offshore muds 
and the muds over earlier deposited oozes. If sufl&cient time has elapsed 
between deposition of offlap beds and the earlier deposited sediments, 
a marine regressive unconformity may be formed. This is likely to be a 
disconformity which might be diflScult to distinguish from conformable 
deposition. 

Other Marine Unconformities: The rise of sea level may be inter- 
rupted by periods of stability during which the sea bottom may be built 
to the base level of deposition for a considerable distance from shore. 
Equilibrium established over a long period of time will result in a dis- 
conformity of nondeposition. 

A falling sea may lower the base level of erosion so that previously 



Stratigraphic, Structural, and Correlation Considerations 



39 



deposited sediments will be removed, and the erosion surface will be- 
come an unconformity should deposition follow this erosion. 

Recognition of Unconformities 

There are a number of signposts that are useful in detecting the 
presence of unconformities. Some of these, such as a basal conglom- 
erate, have become traditions, although experience shows that they may 
be the exception rather than the rule. There are probably more uncon- 
formities without a basal conglomerate than with one. 

Unconformities are best recognized from direct observation in a 
single outcrop. Such an outcrop might be observed in a road cut, a 




2800 feet more of Paleozoic strata 



Bright Angel Shale 



I 
^ I 



X /Archean. Granite/ - 

»:; \'\ i; _ ^ N V / \ x^^ 
^ -^ i ^ , ' \ ^ / , \ , I ' 



Figure 16. Diagrammatic cross section of inner part of Grand Canyon, showing Ep- 
Algonkian and Ep-Archean unconformities. 

quarry, a surface exposure on canyon walls, a ravine, or a cliff. For ex- 
ample, probably the most striking exposure of an unconformity can be 
seen in the Grand Canyon of the Colorado River, in the walls of which 
there are two unconformities exposed. Sharp ^"^ has described them 
thoroughly; the diagrammatic section shown in figure 16 is from his 
article. 

In the subsurface the recognition of unconformities depends on the 
ability of the geologist to recognize any of the many criteria for uncon- 
formity from the sparse evidence afforded by well cuttings, cores, and 

^' Sharp, R. p., Ep-Archean and Ep-Algonkian Eroiion Surfaces, Grand Canyon, Arizona: Geol. Soc. 
Americ?i §un,, vol, 51, pp. 1235-127Q, 194Q, 



40 



Subsurface Geologic Methods 



electric logs. A single criterion suggesting unconformity may only com- 
plicate the problem, as almost all such criteria can be explained by fault- 
ing or some sedimentary process or variation. The more associated criteria 
than can be established for a given horizon, the greater the probability 



TABLE 2 
Criteria for Recognition of Unconformities 



Criterion 



Associated with unconformities 
of indicated origin 



Submarine 



Basal conglomerate - 

Basal black shale 

Desert varnish 

Residual (weathered) chert 

Silicified erosion surface 

Phosphatized erosion surface 

Caliche 

Duricrust 

Porous zones in limestone 

Asphaltic and oil-stained zones 

Buried soil profiles 

Lag gravels (pebble bands) ... 

Glauconite zones 

Iron-oxide zones 

Interbedded conglomerate 

Clastic zones in nonclastics 

Abrupt change in heavy-mineral assemblages. 

Radioactive mineral zones 

Porous zones in general 

Sharp differences in lithology 

Abrupt change in chemical composition 

X-ray patterns in vyrell cuttings 

Concretionary and pisolitic zones 

Abrupt change in fauna 

Gaps in evolutionary development 

Algal biscuits 

Bone and tooth conglomerates 

Undulatory surface of contact 

Edgewise conglomerate 

Phosphatic pellets or nodules 

Manganiferous zones 

Pyritiferous zones 

Corrosion surfaces 

Borings of littoral marine organisms 

Lateral spreading of coral reefs 



XX 
XX 

X 
XX 
XX 

X 

? 

XX 
X 

? 

XX 
X 
XX 
XX 
XX 
XX 
X 
XX 
XX 
XX 
XX 
XX 
XX 
XX 



of its being a surface of unconformity. Pyrite crystals by themselves 
may suggest unconformity, but they might also be a normal dissemination 
through a conformable series. If they are associated with glauconite and 
phosphatic pellets, then a marine unconformity is probably present. 

Krumbein ^^ has tabulated criteria cited by various workers for recog- 
nition of unconformities. Pettijohn ^^ relisted Krumbein's criteria with 



^^ Krumbein, W. C, Criteria for the Subsurf'Sce Recognition of Unconformities: 
troleum Geologists Bull., vol. 26, no. 1, pp. 36-62, Jan. 1942. 

''Pettijohn, F. J., Sedimentary Rocks, p. 147, New York, Harper & Brothers, 1949. 



Assoc. Pe- 



Stratigraphic, Structural, and Correlation Considerations 



41 



modifications as shown in table 2. A common or an exclusive association 
is indicated by xx, an occasional or rare association by x, and an associa- 
tion that probably does not occur by a blank. A number of these criteria 
are included in the subsequent discussion. 

Structural Features. Discordance in Dip : Lack of parallelism in beds 
on both sides of an unconformable contact may be readily observed in 
vertical sections such as the face of a cliff. Discordance alone should not 
be accepted as positive proof without further observation, especially if 
the exposure is a small one. Faulting, contemporaneous erosion, or large- 
scale cross bedding may produce discordance. 

The presence of dip discordance in a sporadically cored subsurface 
section may suggest, but does not prove, angular unconformity. A number 




FACIES CHANGE 



CROSSING AXIAL SURFACE 




FLOWAGE DEFORMATION 



UNDETECTED FAULT 



A 






^^^^ ^ 




^^^ 


^ ~~~\ 


N^\\ 







■ -—, / 


L— --''^*^ — ^--- 


/-^ 




^ 1 


ro^~~~~~— -^^ 


C^<?^vi:^-.' 


lIoTvTT:^-:- 


/.' " • ■ -'. " •->^-^^''' 


~" — '-^^ . • - * 


P=^^^ 


~^^ 



HOLE DEVIATION 



FALSE- BEDDING 



Figure 17. Diagrammatic sketches showing conditions involving spot cores the dip 
variances of which may suggest unconformity. 



42 Subsurface Geologic Methods 

of features that might give dip variation are undetected faults, hole devia- 
tion, cross bedding, flowage, and crossing of the axial surface of a fold. 
Figure 17 illustrates several situations in which a nonconformity would be 
apparent from cores taken at intervals, if angular discordance were the 
only criterion. 

Truncation of Faults or Intrusives: Faults and intrusive bodies in 
older strata may be truncated by erosion and end abruptly at unconform- 
able contacts. There may be a definite contrast in the number of intrusives 
and the faulting present above and below an unconformity. Again caution 
should be used in using this criterion, as faulting may produce the same 
contrasts. 

Degree of Folding or Metamorphism : There may be definite contrasts 
in the degree of deformation or metamorphism on both sides of an uncon- 
formable contact. Faulting again, however, may produce the same result. 

Evidences of Erosion or Weathering. Basal Conglomerate: It was 
previously mentioned that a basal conglomerate may not be present in 
either major type of unconformity. When present, the basal conglomerate 
is composed of the coarser debris of the transgressed erosional surface. 
It may be definitely arkosic if that surface is underlain by granite rocks 
and may even be a blended unconformity if the transgressed surface is 
deeply weathered. 

If it is a marine unconformity, where the older strata were below sea 
level before deposition of the younger, there may be bones, teeth, and shell 
fragments, in addition to pebbles, in the basal formations of the younger 
sediments. 

Autoclastic rocks might be confused with basal conglomerates, for 
example, fault breccias. However, they are usually more angular than con- 
glomeritic pebbles. They will contain material from the rocks on both sides 
of the fault and may be filled with vein material. 

Lag Gravels: Lag gravels are an accumulation of coarse debris left 
after the removal of the finer material by wind or water. They may repre- 
sent an eolian unconformity where concentration has been effected by the 
scouring action of the wind under desert conditions. As such they may con- 
tain wind-polished and faceted pebbles called "ventifacts" or pebbles coated 
with "desert varnish" and may be tightly fitted together from an old 
"desert pavement." 

Wave and current action may also concentrate lag gravels under con- 
ditions of submarine erosion. The presence of pebble bands in a marine 
section suggests a diastem, if found within a formation, or a disconform- 
ity of larger magnitude, if located at the top of a formation. 

Buried Soils: One of the strongest proofs of unconformity is the 
identification of ancient soils in the geologic column. This identification 
may be very difficult because of the changes that take place in soils sub- 
sequent to burial. They may be partly or completely incorporated into the 
basal part of the new formation and may lose the zoned characteristics 



Stratigraphic, Structural, and Correlation Considerations 43 

typical of recent soils while being reworked by an advancing sea. Krum- 
bein ^^ in his excellent article enumerates a number of criteria useful in 
recognizing ancient soil horizons. 

A concentration of iron oxides may be indicative of old soil profiles. 
In the Midcontinent, thin beds of red shale in normal sections of the 
Pennsylvanian are considered to be ancient soil profiles. During the for- 
mation of lateritic soils, there is a strong concentration of iron, which may 
show up as a tough, porous "duricrust." Zones of concretions may be 
found in fossil soil horizons. Caliche, for example, is often concretionary. 
It has been suggested that variegated shales might be the end products of 
modifications in long-buried soils. Basal black shales may be formed when 
soils with a high humus content are incorporated into the first formation 
of a marine transgressive overlap. 

Certain residual deposits should also be considered under this head- 
ing. The weathering of limestones with the selective solution of the cal- 
cium carbonate may leave a residual soil rich in silica, phosphate, or clay. 
Soils formed from the weathering of cherty limestones might be recognized 
by masses of broken and cemented chert fragments. Concentrations of clay 
or calcium phosphates at the top or within a limestone section are good 
signposts of unconformity. 

Porous Limestone: In areas of limestone bedrock, a considerable por- 
osity may be developed by the solvent effect of ground water. Solution ac- 
tion may develop porosity ranging from microvugular to cavernous, de- 
pending on such factors as precipitation, rate of erosion, and length of 
exposure to erosion. Although other agents and processes may be respon- 
sible for limestone porosity, solution channels, caverns, and a decrease in 
porosity in depth without change in lithology are good evidences of ground- 
water action and hence of unconformity. Further, the porosity not only is 
a criterion for unconformity but may also be the locus for commercial oil 
accumulation. 

Angular Coal Fragments: An interesting proof of a long erosional 
break in Pennsylvania sedimentation in eastern Kansas has been ad- 
vanced by John L. Rich.^^ He found angular coal fragments in the lower 
few feet of a channel sandstone in the base of the Lawrence shale near 
Ottawa, Kansas. Many fragments have square ends indicating jointing 
before burial. The coal must have advanced at least to the lignite stage. 
The source of the coal fragments must be from underlying Weston shale, 
also Pennsylvanian, which shows marked crumpling below the uncon- 
formity with the Lawrence shale. If coal of Weston age had become 
lignite before its burial in Lawrence shale, the unconformity between 
these two must represent a long time interval. 

Silicified Shell Fragments: Weathering of some Paleozoic lime- 
stones with subsequent development of residual clay has been observed 

^"Op. cit. 

^^ Rich, J. L., Angular Coal Fragments as Evidence of a Long Time Break in Pennsylvanian Sedimen- 
tation in Eastern Kansas : Geol. Soc. America BuU., vol. 44, no. 4, pp. 865-870, Aug. 31, 1933, 



44 Subsurface Geologic Methods 

to result in a peculiar type of silicification of the included fossils. The 
shells in the limestone matrix are wholly calcareous, while those in the 
overlying residuum are wholly silicified. On the surface of the shells 
occur many of the so-called "beekite rings." These are small doughnut- 
like circlets of bluish-gray to white, opaque to translucent quartz. Find- 
ing of silicifed shell fragments with beekite rings indicates that an ero- 
sional zone has been entered and marks an unconformity in a series of 
limestones even if there is no lithologic evidence. 

Color Contrasts: A sharp contrast above and below an unconformity 
in the colors of the sediments often marks a disconformable contact where 
there is little other evidence. A bright red, yellow, or purple below the 
unconformity indicates possible weathering and oxidation of iron — or 
manganese-bearing minerals. 

Chemical Sediments. Concentrations of chemical sediments on un- 
conformity surfaces may be indicative of erosional breaks or cessation 
of sedimentation. 

Manganese: Many manganese deposits are found associated with 
the basal strata overlying unconformities. Some of these concentrations 
are due to accumulation of residual materials on a weathered surface. 
The oxides of manganese are very stable, and accumulations of them are 
present on deeply eroded surfaces. Other accumulations of manganese 
may be found at the base level of deposition during times of little or no 
deposition of clastic sediments. 

Phosphates: Phosphate nodules are indicative of cessation or ex- 
treme slowness of deposition. The source of phosphorus may be shells 
and other organic material. Marine currents working over a base level 
of deposition may remove parts of the sediments, concentrating the phos- 
phates. Limestones or limy muds are particularly susceptible; the car- 
bonates are removed, and the phosphates are concentrated. 

Glauconite: Glauconite deposits are formed by diagenetic processes 
and require periods of nondeposition or very slow deposition for their 
formation. A concentration of glauconite indicates but does not prove 
unconformity. 

Caliche: Caliche deposits, if identifiable as such in the geologic 
column, are indicative of unconformities. Caliche is formed by the 
evaporation of carbonate-bearing capillary waters that come to the surface 
in semi arid regions. These deposits may be a yard or more in thick- 
ness and conform to the surface. They form on uplands as well as low- 
lands. On submergence, these deposits may be buried beneath a blanket 
of sediments and, if identified, mark an unconformity. 

Pyrite: Pyrite may be concentrated at or near surfaces of discon- 
formity in marine sediments. Precipitation of iron sulphide apparently 
takes place along ocean bottoms where profiles of deposition have been 
established. Pyrite crystals may be disseminated through carbonaceous 
shales and organic limestones where no unconformity exists and are, 



Stratigraphic, Structural, and Correlation Considerations 45 

therefore, not conclusive proof of breaks in sedimentation. If tRey are 
found in association with other chemical sediments such as manganese 
or phosphate nodules or glauconite, an unconformity is strongly sug- 
gested. The presence of pyrite at contacts of contrasting lithologies is 
good evidence of unconformity. 

Fossil Evidence: Probably the most reliable means of detecting un- 
conformities in the geologic column are the index fossils found in forma- 
tions on both sides of the unconformity. Many extensive time breaks in 
sedimentation, especially disconformities, if unaccompanied by erosion 
or deformation, may leave no other proof of their presence in a series of 
stratified rocks. There are probably many unrecognized unconformities 
because of a lack of fossils for comparison. 

Importance of Unconformities 

Unconformities are of paramount value to the stratigrapher in sep- 
arating units of rocks and of geologic time. They are of great importance 
to the oil geologist because of the vast amounts of petroleum that may 
be trapped at or near their surfaces. 

Stratigraphic. Geologists have long used diastrophism as the ulti- 
mate basis for subdividing geologic time and the geologic column. The 
geologic record shows evidence of rhythmic changes in sea level. These 
changes are controlled by recurrent or cyclic periods of disturbance. A 
geologic cycle usually starts with an unconformity and ends with an un- 
conformity. The diastrophism that closes one cycle is usually accom- 
panied by mountainbuilding and deepening of the ocean basins. Not 
only are there wide spread breaks in sedimentation, but vast areas are 
exposed to erosion. As land areas are worn down and sediments are 
carried into the ocean basins, the seas will slowly rise and cover the 
lower parts of the continents with shallow seas. Sediments will be de- 
posited unconformably on the submerged erosion surface. An examina- 
tion of the geologic record shows a great succession of marine inunda- 
tions of the continents separated by unconformities marking the retreat 
of marine waters. Since the oceans are freely connected, a change in sea 
level will affect all, and major breaks will be worldwide. 

The cycles of earth movements do not follow a smooth pattern. Be- 
tween major periods of diastrophism will be irregular periods of dis- 
turbance involving local emergence and submergence of land areas. In 
regions affected by these minor breaks there will be a marked hiatus 
and unconformity. 

Breaks in the Fossil Record: The fossil record affords us the best 
direct means of evaluating the time breaks indicated by unconformities. 
The evolution of plant and animal forms is always greatly accelerated 
during periods of crustal disturbance. Some forms adapt themselves to 
the changes in environment and survive with variations in structure, 
while others fail to adjust themselves and perish. The next invasion of 



46 Subsurface Geologic Methods 

the sea traps within its sediments a new assemblage of fossil flora and 
fauna. Great breaks in the physical records are accompanied by breaks 
in the biologic record. Fossils are, therefore, the most dependable means 
of detecting the time break necessary to establish an unconformable 
break in the geologic column. 

Subdivisions of the Geologic Record: The greatest breaks in the geo- 
logic record result from widespread continental emergence. These may 
be accompanied by large-scale mountain building with much deformation 
of previously formed sediments. These revolutions separate geologic time 
into eras and are marked by worldwide unconformities. Within each era 
are periods of crustal disturbance strong enough to cause widespread re- 
treats of the seas from the continent masses. These produce breaks in the 
geologic record, which are widespread but not universal. The unconformi- 
ties produced by those breaks separate rocks into systems, and the corre- 
sponding time units called periods. Smaller and more local breaks divide 
periods into epochs and systems into series of rocks. 

Oil Reservoirs. A number of conditions may be responsible for oil 
accumulation near unconformable contacts. Soluble rocks such as lime- 
stone when exposed to erosion may become porous by solution. Subse- 
quent deposition of an impervious shale above the unconformable con- 
tact may trap oil in large quantities in the limestone. The West Edmond 
field of Oklahoma produces from the Bois d'Arc limestone member of 
the Hunton formation. An unconformity overlain by Pennsylvanian 
shales seals the oil, which is trapped in truncated Bois d'Arc limestone. 
This limestone was made porous by solution during the weathering of 
the upturned formations. 

Insoluble residual materials may collect on an old erosional plain or 
against the slope of an old high. These accumulations of porous mate- 
rials form potentially good reservoirs. 

The rocks immediately above an unconformity are often shallow- 
water sands and gravels. These may be highly porous and suitable for 
reservoir rocks. 

"Bald-Headed" Structures: "Bald-headed" structures are anticlines or 
domes that have been eroded so that the producing formations are removed 
from the tops of the structures. Subsequent submergence and deposition 
have sealed the truncated formations below the unconformity. Oil is 
found in the flank sands of these structures. The Oklahoma City pool, a 
cross section of which is shown in figure 18, is an excellent example of 
this sort of pool. Simpson formations were stripped from the top of the 
structure by pre-Pennsylvanian erosion. Production from this formation 
was found on the flanks of the structure. The Billings dome in Noble 
County, Oklahoma, was barren at the top owing to erosion of Simpson 
formations. It was not until 17 years later that geologists recommended 
drilling on the flanks of the structure, where large production was found 



Stratigraphic, Structural, and Correlation Considerations 47 

in the "Wilcox" sand. The Nemaha buried ridge of Kansas and Okla- 
homa is perhaps the largest "bald head" of all. 

Shoestring Sands: Very long sand lenses a few hundred feet wide, 
a few score feet thick, and seveial thousand feet to several miles long, 
are called "shoestring sands," when they are found buried in mud de- 
posits or in shale formations. They may be fillings of old stream chan- 
nels or buried offshore bars. If the former, the cross section of the de- 
posit will have its greatest width at the top and will have a base convex 



c 
o 

c 




\> 




o 




.—-^i^^^^^^^^^SJ^-.;-/, • ' 


-7. , .-— ^.^^^ 


_> 




"^IIZ"'^ ' — —-" — \. '•''■•' - 


,'•■. ' _■ ". ^"T'**^«,.__ 


>« 


^^"-""^^^^^^^^^ 


i:::^----^'^^^^^--^^— ~\^^' 


■^-^i:lli'"' ':-'^ ■' - :T^"^>^*-^ 


M 


^^^""^""'^^^^''^ ' '^■'"•'''^— ^.^ 




-^L^^^j*"; . ." * * ; • T^^**>* 


c 


^-■**'^^^''^^--^^^!Z-— — ^""V— ■'"'^'^^ 




^^^"^"^^-ii^.^^^ .-",'' .' ..*■' 


c 


. ■rrr^tr^^^-^^:::^^^^^^;::!!!^^ 




^"^■^-<i\- 


0) 

a. 


^^^^>y/A 




r^^^^^~ ^ 


- ' 


9vWA7///rPy 




\^§^r^ 


c 
o 

o 

> 
o 








■o 

6 






V^^^;r^ 



Figure 18. Idealized section west-east across Oklahoma City field showing relation 
of unconformities to oil and gas production. A typical "bald-headed" struc- 
ture. 

downward. In the latter case, the cross section will have a flat base and 
a top convex upward. These sands will have disconformable relation- 
ships with the surrounding rocks. 

An example of a shoestring-sand oil pool is the Bush City field in 
Anderson County, Kansas. Oil is trapped in a sand body 13 miles long, 
about one-fourth mile wide, and buried 30 to 40 feet below the top of the 
Cherokee shale of Pennsylvanian age. The sand has been folded into 
minor anticlines and synclines. Production is from both because of the 
water-free nature of the sand. A typical cross section and plan of the 
sand is shown in figure 19. 

Disconf ormities : Oil may be found at the surfaces of disconform- 
ities where impervious beds overlie an erosion surface of some relief 
and will be found in structural adjustment in the highs of the older 
strata. Figure 20 shows diagrammatically how petroleum has been trapped 
in hills of the Wilcox formation buried under the impervious clays of the 
Cane River formation; both are Tertiary. 

Regional Unconformities: Unconformities whose surfaces can be 
traced over wide areas are termed "regional unconformities." The inter- 
section of two unconformable surfaces against the flanks of the Sabine 



48 



Subsurface Geoiogic Methods 









/ 


— 


-540 
2 550 

-nan 


























>t '"••".".•" '•..' 


x^ —600 








X^, / —610 
X. . y —620 

^^ -|I8 

—650 
A 
















T 

20 

S 

s 
































^ 




i 




^ 


w 










A 


W 








i 


r 




















J— - 








J 


f 


















A 


^ 




« 


^^ 


B 


4 

~ 


r 




























^ 


*■ 






























































































































RI9E R20E R2IE 


B 



Figure 19. "Shoestring-sand" type oil field; Bush City field, Anderson County, 
Kansas. Cross section in A shows flat top and bottom convex downward of a 
typical stream-channel deposit. Plan in B shows windings of an ancient stream 
channel. 



uplift are responsible for the vast quantities of oil in the East Texas 
field. The Woodbine formation of Upper Cretaceous age has an uncon- 
formity at the top of the formation and is overlain nonconformably by the 
Eagle Ford in the Mexia and Balcones fault zones and by the Austin chalk 
in east Texas (fig. 21). The base of the Woodbine is marked by another 
unconformity, below which are beds of the Comanche group. These two 
unconformities are of regional extent and intersect on the flank of the 
Sabine uplift in east Texas. Oil from the Woodbine is found in the fault 
zone, where it is trapped at a fault surface and below the upper uncon- 
formity. Figure 21 is an idealized west-east section through the Tyler 




Figure 20. Idealized diagram of Urania field, Louisiana, where production comes 
from oil trapped in the highs of old erosion surface disconformably overlain by 
impervious shales. Both the Cane River and Wilcox formations are Eocene. 



Stratigraphic, Structural, and Correlation Considerations 49 

basin showing the function of a regional unconformity in localizing pro- 
duction in the Woodbine sand. 

Major oil structures are often completely obscured by nonconform- 
able deposition of later sediments. The geologic history of the older 
rocks may have been such that great oil and gas accumulations were formed 
below regional unconformities. For example, the Central Kansas uplift is 
obscured by a uniform cover of north- and west-dipping Permian and 




Figure 21. Idealized section west-east of Woodbine sand in Tyler basin of northeast 
Texas showing relation of unconformities to production. In Mexia field to west, 
production is from oil trapped between a fault and an unconformity. In East 
Texas field to east, two unconformities intersect on flank of Sabine uplift to 
form oil trap. This section illustrates importance of regional unconformity to 
production of oil. 

Cretaceous rocks. Again, the Bend arch of Texas is obscured by overlap- 
ping and west-dipping Pennsylvanian strata. 

Two areas offer great possibilities for new geologic conditions con- 
cealed below unconformities. The Comanche rocks of northern Louisiana, 
northeast Texas, and southern Arkansas, where 5,000 to 10,000 feet of 
sediments are folded, tilted, and completely overlapped unconformably 
by Upper Cretaceous rocks. Pre-Carboniferous rocks of west Texas have 
been folded and eroded with great lateral changes in porosity and are 
widespread beneath a cover of Carboniferous rocks. 

Ore Deposits 

Types of deposits that may be found associated with unconformities 
are residual and placer ores. Surfaces of unconformity may also in- 
fluence the localization of hydrothermal deposits. 

Residual Deposits: Erosion surfaces of regional extent may be the 
loci for residual deposits of bauxite, manganese ores, lateritic iron ores, 
and phosphates. If this surface is then buried completely or partly, an 
unconformity results, the tracing and mapping of which may lead to the 
discovery of commercial concentrations of ore. 



50 Subsurface Geologic Methods 

The bauxite deposits of the southern Appalachian states ^^ are an 
example of residual accumulations on a partly buried erosion surface. 
These deposits were formed during the Eocene when the climate was fa- 
vorable for lateritic weathering. The margins of the Appalachians had 
been leveled by the Highland Rim peneplain, with a karst topography de- 
veloping in areas of limestone outcrop. Bauxite accumulated in the sink- 
holes and was incorporated into the basal beds of the Wilcox formation. 
The deposits are now located on remnants of the dissected peneplain, at 
the unconformity between the Wilcox and the older truncated rocks. 

Placer Deposits: Accumulations of placer minerals will often be 
found at or near local unconformities because of their mode of concen- 
tration. Gravels containing heavy minerals are deposited along the reaches 
of a stream where there is slack water. Because of higher specific gravity 
and aided by the jigging action of eddies and swirls, the placer minerals 
settle toward the bottom of the stream channel. The bottom gravels close 
to bedrock will have the richest pay streaks. If the bedrock is rough and 
irregular, natural riffles will trap especially rich streaks. If the stream 
channel is later filled and abandoned, the contact between the dense bed- 
rock and the loose gravels becomes a local unconformity. 

The "high level" Tertiary, auriferous gravels of the Sierra Nevada 
described by Lindgren ^^ are an excellent illustration. Buried Tertiary 
gravels containing placer gold are exposed high on the sides of Quater- 
nary valleys. These gravels were deposited in stream channels cut in 
bedrock during the Eocene, with the richest pay streaks in the deep gravels 
two to three feet off the bottom. The stream channels were then buried by 
lean gravels and volcanics which completely filled the valleys. After 
elevation of the Sierra Nevada in the Pliocene and Pleistocene, new streams 
eroded canyons an average of 2,600 feet below the early Tertiary stream 
bottoms. The gold-bearing gravels have been worked by drift mining and 
hydraulic procedure. 

Hydrothermal Deposits: The influence of unconformities in control- 
ling ore formation by hydrothermal solutions is that of modification and 
not primary control. A review of the literature revealed no districts or 
mines in which an unconformity was considered the primary control in ore 
deposition. Unconformities, if present in the section, were rarely consid- 
ered as having any modifying influence at all. 

Bell ^^ mentions the apparent eff"ect of the slope of an unconformable 
surface on ore deposition in the Hallnor mine, second-largest mine in the 
east Porcupine area of Ontario. An angular unconformity of considerable 
erosional irregularity separates a series of younger sediments from older 
lava flows. A factor that has proved useful in prospecting for new ore 
zones is the slope of the lava-sediment contact. At the west end of the 



^McKinstry, H. E., Mining Geology, New York City, Prentice-Hall, Inc., 1948. 
^3 Lindgren, W., Tertiary Gravels of the Sierra Nevada, U. S. Geol. Surv. Prof. Paper 72, 1911. 
^^ Bell, A. M., Hallnor Mine, Structural Geology of Canadian Ore Deposits: Canadian Inst, of Min. 
Metallurgy Trans., 1948. 



Stratigraphic, Structural, and Correlation Considerations 51 

property at the upper levels, a hill of lava projects stratigraphically into 
the sediments and is marked by a vertical contact. In the lower levels the 
contact flattens where an embayment of sediments cuts into the old lava 
surface. The No. 1 vein system occurs opposite the steep contact. Below 
this the ore is absent along the flatter-dipping contact but recurs where the 
unconformity starts to steepen. 

Conclusion 

Because of the stratigraphic and economic value of unconformities, 
the geologist should become adept in using the criteria by which uncon- 
formities may be detected. Undiscovered reservoirs of petroleum of the 
magnitude of the East Texas field may lie beneath the anonymity of 
regional unconformities on the Gulf Coast, in west Texas, and in some of 
the Rocky Mountain States. Unconformities may prove to be important 
factors in controlling and localizing ore deposition in many mining areas 
where their potentialities have been ignored. 

Faults 

Geologists are aware of the need of fault control before stratigraphic 
problems can be solved. Fault patterns and their relationships may be 
extremely complicated (fig. 22) and may play such a role that both sur- 
face and subsurface structural and stratigraphic trends can be evaluated 
only after voluminous data become available for interpretation. 

A fault represents a surface along which one rock segment has moved 
with respect to the other. The magnitude of faults ranges from millimeters 
of displacement to several thousands of feet or even miles. Faults are 
developed in all rock types; their displacement may increase or decrease 
with depth or vice versa, and their relative movement may be vertical, hori- 
zontal, or rotational. 

The apparent movement along a fault is a function of many variables, 
and depends not only on the net slip, but also on the strike and dip of the 
fault, the dip and strike of the disrupted stratum, and the attitude of the surface 
on which the observations are made.'*^ 

Common fault types include strike faults, in which the strikes are 
more or less parallel to the strike of the rocks involved; oblique faults, 
in which the strikes are diagonal to the strike of the rocks involved; 
longitudinal faults, in which the strikes are roughly parallel to the strike 
of the regional structural fabric; and transverse faults, in which the strikes 
are perpendicular or oblique to the strike of the regional structural fabric. 
Other fault varieties are en echelon, peripheral, and radial. High-angled 
faults are those with surfaces that dip greater than 45 degrees. Low-angled 
faults dip less than 45 degrees. 

A thrust fault (reverse) or thrust is a fault along which the hanging 
wall has moved up relative to the footwall. A gravity fault (normal) is a 

^ Billings, M. P., Structural Geology, p. 137, New York, Prentice-Hall, Inc., 1942. 



52 



Subsurface Geologic Methods 



fault along which the hanging wall has moved down relative to the foot- 
wall.46 

Billings ^^ lists the following criteria to aid in the recognition of 
faults: (1) the discontinuity of structures, (2) the repetition or omission 
of strata, (3) features characteristic of fault planes, (4) silicification and 
mineralization, (5) sudden changes in sedimentary types and (6) physio- 
graphic data. Only certain of these criteria may be applied in evaluating 










m^mmMiif- 



Wafer iai 



..a. 



/7 ^- <. 












Figure 22. Complexly faulted Miocene shales exposed in a road cut in Grimes 
Canyon, Ventura County, California. This sketch clearly demonstrates complex 
structural conditions which may develop in the subsurface as a result of thrust 
faulting. Note structural relations above and below thrust zone. Decipherment 
of this type of geologic problem requires close coordination between the struc- 
tural and stratigraphic geologists. Drawing was sketched from a projected 
35-mm. Kodachrome slide. Graphic symbols used do not represent true lithology 
but illustrate only minor compositional variations in the shale section. 



the presence of a fault in the subsurface. To establish the type and char- 
acter of faulting in well sections the following indications have proved 
applicable: (1) anomalous profiles of lithologic, electric, and radioactive 
logs; (2) a change in dip and disturbed phases as exhibited by cores; (3) 
mineralogic and paleontologic irregularities; (4) lost circulation; (5) a 
caving hole; (6) an increase in penetration rate; (7) an increase in por- 
osity and permeability; (8) slickensided fragments in ditch samples; (9) 
a sudden increase of drilling-mud temperature; (10) poor core recovery; 
and (11) an abrupt deviation of the hole from the vertical. 

" Billings, M. P., op cit., p. 152. 
" Billings, M. P., op. cit., p. 155. 



Stratigraphic, Structural, and Correlation Considerations 



53 



Many subsurface faults are not reflected at the surface owing to 
"dying out" or to truncation by buried erosional surfaces (fig. 23). Con- 
versely, faults intersecting the surface may be absorbed by incompetent 
beds in the subsurface and thus may have restricted vertical downward 
extension. 

It should be recognized that faults involve surfaces and not planes. 
Many irregularities and curvatures which these surfaces may assume make 
their position and trend in the subsurface difficult to define. These ir- 






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■ ■ ■/ //^^'-r-C^ '•.'■.■:•■.••■.■ '• T^TT 


rr^tr^ 


^^f^ 


^? -^ 


-^i^ 


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ili 



Figure 23. Various types of faults. A — fault surfaces may be extremely irregular; 
the proper interpretation of details of fault zones in the subsurface requires con- 
siderable data. B — Many faults disappear with depth owing to displacement 
absorption by incompetent strata. C — Surface folds may be replaced by vertical 
displacement with depth. Z) — Fault systems may be truncated by erosional sur- 
faces. E — Favorable petroliferous structure may underlie overthrust sheets. F — 
Major thrust faulting may create accompanying tensional fault patterns. 







gi2 


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Stratigraphic, Structural, and Correlation Considerations 55 

regularities complicate and, in many instances, control the accumulation 
pattern of oil and gas and other concentrations of minerals. Fault rela- 
tionships should always be viewed in three dimensions. Fault surfaces 
may be contoured and fault gaps outlined.^^ 

Marine Onlap and Offlap 

Transgressions and regressions of the sea across continental segments 
produce sedimentary onlaps and oflflaps. These depositional features 
are of local or regional magnitude and have been responsible in many 
areas for conditions permitting the accumulation of large quantities of 
oil and gas. Problems of onlap and offlap of varying complexity are com- 
monly encountered in stratigraphic work. 

Patterson ^^ has clearly demonstrated the transgressive and regressive 
relationships of the Cook Mountain, Yegua, and Fayette formations (upper 
Eocene) developed across Webb, Zapata, and Starr Counties, Texas. These 
relationships are idealistically shown in figure 24. 

Swain's definitions ^° of various terms relating to transgressions and 
regressions follow : 

1. Onlap: The progressive pincliing-out, toward the margins of a deposi- 
tional basin, of the sedimentary units of a conformable sequence of rocks. 

2. Offlap: The progressively offshore degression of the updip termina- 
tions of the sedimentary units of a conformable sequence of rocks. 

3. Overstep: The regular truncation of older units of a complete sedi- 
mentary sequence by one or more later units of the sequence. The resulting 
unconformity may be either marginal to the basin of deposition, or within the 
basin as a result of local uplifts. If more than one unit rests on those beneath 
the unconformity, both overstep and onlap are involved. 

4. Complete overstep: The entire blanketing with unconformable rela- 
tionship of the older rocks of a basin by younger rocks. 

To obtain the true trend and magnitude of marine onlaps and offlaps, 
one must give full consideration to the third-dimensional concept of the 
region. 

Oil and Gas Traps 

Oil and gas accumulations (fig. 25) occur under favorable structural, 
stratigraphic, and structural-stratigraphic conditions. Examples of various 
types of entrapments are graphically illustrated in figures 26-32. Pros- 
pective oil and gas reservoirs must be considered from a three-dimensional 
viewpoint. The closure factor should under no circumstances be min- 
imized. To evaluate closure, whether it be controlled by structural or 
stratigraphic conditions or a combination of both, requires the interpre- 
tative ability of both the surface and subsurface geologist. 

•"*Reiter, W. A., Contouring Fault Planes: Oil World, July 14, 1947. 

^^ Patterson, J. M., Stratigraphy of Eocene between Laredo and Rio Grande City, Texas: Assoc. Pe- 
troleum Geologists Bull., vol. 26, no. 2, pp. 256-274, Feb. 1942. 

^^ Swain, F. M., Onlap, Offlap, Overstep and Overlap, Am. Assoc. Petroleum Geologists Bull., vol. 
33, no. 4, pp. 634-635, 1949. 



56 



Subsurface Geologic Methods 



Clapp ^^ in 1910 proposed the first detailed classification of natural 
oil and gas accumulations and modified "- this classification in 1917. Fi- 
nally in 1929 ^^ he presented the following classification: 



I. 



II. 
III. 



IV. 



Anticlinal structures 

1. Normal anticlines 

2. Broad geanticlinal folds 

3. Overturned folds 
Synclinal structures 
Homoclinal structures 

1. Structural "terraces" 

2. Homoclinal "noses'" 

3. Homoclinal "ravines" 
Quaquaversal structures (domes) 

1. Domes on anticlines 

2. Domes on homoclines and monoclines 

3. Closed salt domes 

4. Perforated salt domes 




YimL 



Gas 



Water 



A,B,C.D -- Wells 



Figure 25. Diagram showing irregular distribution of gas, oil and water in an asym- 
metric anticline. Such relationships require orderly development of field to 
insure maximum and efficient recovery. Rate and pattern of encroachment of 
edge and bottom waters are of primary concern to operators. 



^^ Clapp, F. G., A Proposed Classification of Petroleum and Natural Gas Fields: Econ. Geo!,, vol. 5, 
pp. 503-521, 1910. 

*^ Clapp, F. G., Revision of the Structural Classification of Petroleum and Natural Petroleum and 
Natural Gas Fields: Geol. Soc. American Bull., vol. 28, pp. 553-602, 1917. 

*' Clapp, F. G., Role of Geologic Structure in the Accumulation of Petroleum, in Structure of Typical 
American Oil Fields, II: Am. Assoc. Petroleum Geologists Bull., pp. 671-672, 1929. 



Stratigraphic, Structural, and Correlation Considerations 57 

5. Domal structures caused by igneous intrusions 
V. Unconformities 
VI. Lenticular sands (on structure) 
VII. Crevices and cavities irrespective of other structure 

1. In limestones and dolomites 

2. In shales 

3. In igneous rocks 
VIII. Structures due to faulting 

1. On upthrowTi and downthrown sides 

2. Overthrusts 

3. Fault blocks 

Wilson °^ in 1934 proposed a very logical and well-organized classi- 
fication based on local deformation of strata, variation of rock porosity, 
and combinations of the two. His classification is as follows: 

I. Closed reservoirs 

A. Reservoirs closed by local deformation of strata 

1. Reservoirs closed by folding 

a. Reservoirs in closed anticlines and domes 

b. Reservoirs in closed synclines and basins 

2. Reservoirs developed by offsetting of strata by faulting of 
homoclinal structure 

3. Reservoirs defined by combinations of folding and faulting 

4. Reservoirs formed through disturbance of strata by intrusions 

a. Intrusions of salt 

b. Intrusions of igneous rock 

5. Reservoirs developed in fault and joint fissures and in crushed 
zones 

B. Reservoirs closed because of varying porosity of rock. No de- 
formation of strata required other than regional tilting 

1. Reservoirs in sandstone caused by lensing of sandstone or by 
varying porosity in sandstone 

2. Lensing porous zones in limestones and dolomites 

3. Lensing porous zones in igneous and metamorphic rocks 

4. Reservoirs in truncated and sealed strata 

a. Closed by overlap of relatively impervious rock 

b. Closed by seal of viscous hydrocarbons 

C. Reservoirs closed by combination of folding and varying porosity 

D. Reservoirs closed by combination of faulting and varying porosity 
II. Open reservoirs 

None of economic importance. 

Sanders ^^ proposed a broad subdivision of trap types: (1) structural 
traps, (2) Stratigraphic traps, and (3) combinations of structural-strati- 
graphic traps. Heroy ^^ recognized four groups of oil and gas traps: (1) 
depositional traps, (2) diagenetic traps, (3) deformational traps and (4) 
combination traps. 

In 1945 Wilhelm ^^ in a very worthy discussion, subdivided petroleum 



^'Wilson, W. B., Proposed Classification of Oil and Gas Reservoirs, Problems of Pet. Geol. (Tulsa, 
Am. Assoc. Petroleum Geologists, pp. 433-445, 1934). 

^ Sanders, C. W., Stratigraphic Type Oil Fields and Proposed New Classification of Reservoir Traps, 
Am. Assoc. Petroleum Geologists Bull., vol. 27, pp. 539-550, 1943. 

"8 Heroy, W. B., Petroleum Geology, Geology, 1888-1938, Fiftieth Anniversary Volume, Geol. Soc. 
America Bull., pp. 534-539, 1941. 

°' Wilhelm, O., Classification of Petroleum Reservoirs, Am. Assoc. Petroleum Geologists Bull., vol. 
29, pp. 1537-1579, 1945. 



58 



Subsurface Geologic Methods 




D . H ' 

Figure 26. A — Accumulation in porous beds below an erosional surface overlain by 
impervious strata. A common variety of fault and lens trap is also shov\rn. The 
reservoirs represent both stratigraphic and stratigraphic-structural types. B, C, 
D, E — Accumulations attributed mainly to structural control. F, G— Structural 
and stratigraphic-structural relationships that account for accumulations. H — 
Accumulation that has been controlled by structural conditions. 



Stratigraphic, Structural, and Correlation Considerations 



59 










^^K 


§1 




^fe^% 


s^^ 





D H 

Figure 27. Various types of structural traps; combination of folding and faulting 
involved. G illustrates migration of the crestal high as a result of convergence 
of section. 



60 Subsurface Geologic Methods 

reservoirs into five major groups: (1) convex-trap reservoirs, (2) per- 
meability-trap reservoirs, (3) pinch-out traps reservoirs, (4) fault-trap 
reservoirs and (5) piercement-trap reservoirs. Examples for these types 
are given in figures 31 and 32. 

In exploiting a potential area for oil and gas the geologist is expected 
to consider the factors suggested as outlined below: 

I. Source rock 

(1) What rock types would probably serve as the most favorable source, 
and what percentage of these types constitutes the total section? 

(2) What are the sedimentational and structural relationships between 
possible source strata, reservoir strata, and entrapment conditions? 

(3) Is permeability sufficient to permit migration from the source strata 
to associated porous types? 

(4) What is the thickness, organic variation, and distribution? 

II. Reservoir rock 

(1) What rock types are most favorable (sandstone, fractured limestone 
or shale, cavernous limestone and dolomite, etc.) ? 

(2) What relationships exist between the reservoir rock and the general 
and local structural and stratigraphic fabric? 

(3) Could the strata qualify as both a source and a reservoir rock? 

(4) What about thickness, distribution, and uniformity of lithology and 
permeability? 

(5) What are the time and accumulation relationships to erosional sur- 
faces ? 

(6) Are the strata within reach of economic drilling? 

III. Entrapment conditions 

(1) Are the entrapments controlled by sedimentational or structural irreg- 
ularities or both? If go, to what extent? 

(2) What are the relationships between the trap feature, the source rock, 
and the reservoir? 

(3) What is the extent of the trap? 

IV. Geologic history 

(1) What is the sedimentational history of the area? 
(a) Period and extent of oscillations. 

(6) Sedimentational trends developed during oscillation stages. 

(c) Source direction of sediments. 

(d) Diagenetic changes. 

(e) Ratio and variations of lithologies in the section. 

(2) What is the structural history of the area? 

(a) What effect did structural adjustment during sedimentation have 

on the development of the stratigraphic pattern? 
(6) When did the major periods of folding and erosion occur? 

Evaluation of the preceding questions may not be based entirely on 
surface data. In this case, it becomes the responsibility of the subsurface 
geologist to integrate information obtained from drilling and from geo- 
physical results. 

Correlation Considerations 

There is constant demand from the stratigraphic geologist for ac- 
curate evaluation of sedimentary units, their lithologic, paleontologic, 




D H 

Figure 28. A, B — Structural traps. C — Differential compaction over buried compe- 
tent rocks, which produces flexures in overlying strata. The reservoirs shown are 
controlled by both the structural and stratigraphical factor. D, E — Combination 
of structural and stratigraphic conditions accounting for the accumulations. F — 
Salt intrusions commonly arch the overlying strata thus permitting accumulation 
of oil and gas at various points in the structure. G — Igneous intrusions into 
sedimentary sections, which sometimes induce secondary permeability in the host 
strata and develop favorable reservoir conditions. H — Progressive overlap which 
gives rise to favorable stratigraphic reservoir traps; accumulation may be 
erratically distributed. 



62 Subsurface Geologic Methods 

structural, and correlation values. These evaluations have been greatly 
improved through the introduction, application, and coordination of many 
new and revised techniques and through a more orderly and systematic ap- 
proach to subsurface investigations. 

Correlation, as commonly implied, consists in matching up similar 
lithologies, faunas, and floras. In certain instances this is correct; other 
correlations, however, require the establishment of proof that deposits of 
given characteristics are the time equivalent of contiguous deposits exhib- 
iting entirely diff"erent lithologic and paleontologic aspects. Strata involv- 
ing similar or even identical features do not necessarily indicate age con- 
temporaneity. For example, an environment in a stratigraphic sequence 
may have given rise to a particular lithology and microfauna; under sim- 
ilar conditions these characteristics may occur stratigraphically higher or 
lower in the section. These deposits with their lithologic and paleontologic 
similarities may be correlated only on the basis of environment and not 
on the basis of equivalent time. Such correlations have been made in the 
past and are now being made, and they have led to unnecessary or even 
disastrous development and exploration recommendations. 

To exemplify the preceding comments, one has only to review condi- 
tions prevailing along modern coast lines, where many unlike though con- 
temporaneous sedimentary and biologic realms are evident. Twenhofel ^^ 
emphasizes this point by commenting: 

. . . When two deposits of the geologic column have been found to hold 
pretty much the same organisms, it has been assumed that the two deposits 
have synchronous relations. It is equally if not more valid to assume that the 
two deposits were laid down under similar environments and may actually be 
somewhat different in age. 

Such anomalies have prevailed throughout geologic time. 

As previously mentioned (p. 15), three primary types of correlations 
must be considered in stratigraphic geology: time, time-rock, and rock. 
Time correlations involve time span only and are based on time-rock di- 
visions. Time-rock or time-stratigraphic correlations involve correlation 
of variable rock types that accumulated during intervals of time. Rock 
or lithogenetic correlations are based essentially on lithologic constitu- 
tion. Boundaries of the last two units may transect or coincide (fig. 6) . 
Neglect of defining these basic stratigraphic units only introduces chaos 
to the science of stratigraphy. 

Some correlations, particularly those involving the time-rock type, 
generally, require voluminous data before being satisfactorily and ade- 
quately established. This problem is most critical in areas in which the 
strata exhibit extreme and rapid vertical and lateral facies changes. In 
areas where stratal sequences are reasonably uniform and constant, as in 
certain parts of the Paleozoic section of the Midcontinent region, correla- 

°* Twenhofel, W. H., Report of Committee on Paleoecology, Nat. Research Council, Oct. 1935. 



Straticraphic, Structural, and Correlation Considerations 63 




3 








D H 

Figure 29. A, B — Primary and secondary porosity and permeability frequently found 
in limestones and dolostones. Such conditions favor oil and gas concentrations. 
These accumulations are difiBcult to discover and develop and their reserves diffi- 
cult to predict. C-F — Typical stratigraphic traps. C — progressive overlap. D — 
unconformity. E — updip lensing. F — sand lentils. G — Accumulation in fold frac- 
tures of relatively impermeable strata. H — Updip tar seal. 



64 Subsurface Geologic Methods 

tions may be accurately extended over appreciable distances without undue 
concern. 

Methods of correlation vary according to stratigraphic and struc- 
tural complexity (whether general or detailed results are desired, whether 
the problem is of surface or subsurface variety, or both) the time and ex- 
penditure allotted to the assignment, the quality of the personnel, and 
the policy of the management. 

Purposes of Correlation 

The correlation of rocks is the foundation upon which the history 
of the earth can be deciphered. Correlations result in (1) constructing 
composite geologic sections; (2) deciphering surface and subsurface con- 
ditions; (3) coordinating surface and subsurface sequences; (4) evaluat- 
ing contemporaneous and noncontemporaneous rocks; (5) interpreting 
geologic history; (6) identifying and evaluating unconformities; (7) in- 
terpreting environmental conditions and variations; (8) evaluating iso- 
pachous and lithofacies data; (9) identifying outliers; (10) exploring 
and developing natural resources; and (11) making recommendations for 
well locations, casing points, testing, and abandonment of wells. 

Methods of Correlation 

Numerous methods of varying complexity and simplicity are em- 
ployed in correlating sedimentary strata. Techniques applied in subsur- 
face work may diverge widely from those applied in surface investigations. 
The more commonly used correlation procedures are as follows: 

1. Tracing formations or key beds from one locale to another. Strat- 
igraphic relationships of subjacent and superjacent strata should be care- 
fully analyzed when this method is used. 

2. The establishment of control lithologic and paleontologic se- 
quences. 

3. The application of air photographs supplemented by ground in- 
vestigation. Frequently structural and stratigraphic trends that are incon- 
spicuous or unobservable on the ground may be observed on air photo- 
graphs. Photographs may often be used to fill in isolated areas between 
well-controlled stratigraphic sections. 

4. The establishment of erosional surfaces. 

5. The use of paleoclimatic data. These have been found adaptable 
to correlation work in parts of the geologic column. 

6. The application of other techniques, which include chemical, in- 
soluble-residue, specific gravity, stain, detrital-mineralogy, screen, spec- 
trographic, paleontologic, porosity and permeability, settling-rate, and 
X-ray analysis. 

7. The use of well-logging methods, such as electric, radioactive, 
thermal, caliper, mud, and drill-time. These methods have greatly as- 
sisted in establishing more reliable correlations in the subsurface. 




Figure 30. A — Accumulation on flank of an anticline as a result of crestal cementa- 
tion of producing strata. B — Trap resulting from terracing and updip cementa- 
tion. C — Anticlinal accumulation of an overthrust sheet. D — Structural accumu- 
lation in the npdip nonmarine section. E — Accumulation along an unconformity 
and in solution cavities within a carbonate section. F — Anticlinal accumulation 
above a non-piercement salt mass. G — Accumulation in a complexly folded re- 
gion. H — Accumulation in thrust-fault areas. 



66 



Subsurface Geologic Methods 













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11 1 i* ^^ 


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Figure 31. Plan views of oil concentration. Numbers 1 and 2 represent pinch-out- 
trap reservoirs. Numbers 3, 4, 5, and 6 are typical fault-segment reservoirs. 
Numbers 7 and 8 are salt-piercement type reservoirs. Wilhelm's reservoir classi- 
fication indices are given below each drawing. (Adapted from Wilhelm. Am. 
Assoc. Petroleum Geologists.) 



Stratigraphic, Structural, and Correlation Considerations 



67 




B-ni 



B-3Z: 



Figure 32. Plan views (except 2 and 4) of oil concentration. Numbers 1 and 2 are 
simple convex trap reservoirs. Numbers 5, 6, 7, and 8 are permeability trap 
reservoirs. Wilhelm's reservoir classification indices are given below each draw- 
ing. (Adapted from Wilhelm. Am. Assoc. Petroleum Geologists.) 



68 Subsurface Geologic Methods 

In certain instances, depending on the nature of the stratigraphic 
section and the magnitude of the problem, a single method may produce 
the desired correlation results. In others, a combination of one or more 
techniques might be required to obtain the correct solution of the prob- 
lem. It is the responsibility of the stratigrapher to know what method to 
use and when to use it and to recognize the limitations of each. Failure 
to recognize these limitations has often contributed to excessive explora- 
tion and development expenditure. 

Magnitude of Correlations 

Correlations may be of local, regional, interregional, or interconti- 
nental magnitude. 

Local correlations involve those within a restricted area, as for ex- 
ample, an oil field or a minor basin of deposition. When suflBcient data 
are available, correlations of this category are generally not excessively 
difficult, although there are exceptions. When extremely detailed correla- 
tions are required, difficulties are more frequently encountered. 

Regional correlations involve those between separated depositional 
basins within a major province; for example, between the Los Angeles, 
Ventura, Humboldt, and San Joaquin Valley basins of the Pacific Coast 
province or between the various structural basins of the northern Rocky 
Mountain area. Correlations among such basins are of variable complex- 
ity. Certain parts of the section may offer little difficulty in correlation, 
whereas in other parts of the section it may prove impossible to establish 
accurate correlations. 

Interregional correlations, as between the Gulf, Pacific, and Atlantic 
Coast areas, may be moderately dependable; others may be extremely in- 
adequate and dubious. Owing to "far removal" and to facies variations, 
correlation problems of this category may be extremely involved and must 
be paleontologically controlled. 

Intercontinental correlations involve those between continents; for 
example, between the Pacific Coast and the Paris Basin, or between the 
Netherlands East Indies and the British West Indies. Lithologic correla- 
tions cannot be considered. Correlations must be based entirely on pale- 
ontologic data, which, although not very satisfactory, are generally con- 
sidered the most dependable. Certain geologic periods have been fairly 
well established the world over; however, smaller time intervals such as 
the Pliocene, Miocene, Oligocene, and Eocene are open to question. The 
more extended the correlation and the less the time interval involved, the 
more inexact is the chronologic value of the correlation. 

Correlation Indicators 

The selection of correlation indicators depends on the magnitude and 
character of the lithologic and paleontologic aspects of rock and time- 



Stratigraphic, Structural, and Correlation Considerations 69 

rock units. Some indicators applicable for local or even regional corre- 
lation work include coprolites; fish teeth and scales; pollen; spores; 
ooliths; chert phases; glauconite, bentonite, and ash layers; detrital min- 
erals; limestone, coal, and anhydrite beds; cyclothems; and lithic and 
paleontologic sequences. For interregional and intercontinental correla- 
tions pelagic Foraminifera, fusulinids, orbitoids, and ammonites have been 
used with varying success. Evolutionary trends in certain species and 
genera are notably applicable in certain long-range correlation problems. 

Correlation Difficulties 

Some of the more commonly encountered difficulties in correlation 
work are (1) the discontinuity of outcrops; (2) lateral variations in thick- 
ness and lithology; (3) the interval variation between key strata; (4) the 
presence of unrecognized unconformities and faults; (5) the lack of lith- 
ologically and paleontologically controlled sections; (6) the multiplicity 
of time-rock and rock nomenclature; and (7) erroneously compiled and 
interpreted data obtained from the literature. 

Upon entering a new area, these factors should be carefully analyzed. 
All possible lithologic, paleontologic, and structural data should be 
screened and coordinated with the intention of defining intervals and sur- 
faces which will foster improved correlations. 

Questions 

1. Define stratigraphy. 

2. Distinguish between microstratigraphy and macrostratigraphy. 

3. What are the purposes of the American Commission on Strati- 

graphic Nomenclature? 

4. Define "time unit," "time-rock unit," and "rock unit." 

5. Carefully read Hedberg's statement pertaining to stratigraphic 
units. 

6. On what basis may "time surfaces" be defined? 

7. What considerations are necessary for naming a sedimentary for- 
mation? 

8. What recommendations are made by the American Commission on 
Stratigraphic Nomenclature for naming and defining subsurface 
units? 

9. Define "sedimentary facies" and "lithofacies." 

10. What is the importance of evaluating sedimentary-facies changes? 

11. What are the two major types of unconformities and what im- 
portance is attached to these features in stratigraphic geology? 

12. Give ten criteria for recognition of unconformities. 

13. Faults may be recognized on what criteria? 

14. What is meant by marine onlap and off lap? 

15. Study carefully the schematic diagrams illustrating the various 
types of oil and gas traps. 



70 Subsurface Geologic Methods 

16. In exploiting a new area for oil and gas, what major factors 

should the geologist consider? 

17. For what purposes are correlations of sedimentary rocks made? 

18. Give five basic procedures followed in correlating sedimentary 

rocks. 

19. Discuss briefly "magnitude of correlations." 

20. Give five difficulties commonly encountered in correlating data. 



CHAPTER 3 
COMMENTS ON SEDIMENTARY ROCKS 

L. W. LeROY 

No attempt is herein made to present a complete synopsis of the 
major types of sedimentary rocks. However, brief mention of the various 
methods of study applicable in evaluating each type is made. 

The classification of sedimentary rocks is difficult because of inter- 
gradations of textures and compositions. Various classification outlines 
have been proposed and suggested, though none are fully complete. In 
general these classifications fall into two categories: descriptive and 
genetic. The former involves classification without knowledge of origin, 
whereas the latter requires data concerning origin. Neither approach can 
be completely divorced from the other. Two workable classification charts 
are given, one by Van Tuyl ^ (fig. 33) and one by Shrock ^ (fig. 34) . 

For a comprehensive and monumental dissertation on sedimentary 
rocks, Pettijohn's Sedimentary Rocks, published by Harper and Brothers 
in 1949, should be consulted. 

Types of Sedimentary Rocks 
Conglomerates and Breccias 

A conglomerate or its unconsolidated equivalent (gravel) is com- 
posed mainly of rounded granules, pebbles, and boulders exceeding two 
millimeters in diameter. Roundness, sphericity, and flatness of these com- 
ponents show considerable variation. An accumulation of fragments ex- 
hibiting high angularity is commonly termed a "breccia." 

Stratification and degree of sorting shown by the coarse elastics range 
from poor to excellent. Cross-lamination and imbrication patterns are 
frequently developed. 

Conglomerates and breccias have a wide range in color which is 
controlled by the type of finer matrix, the composition of the fragments, 
and the degree of weathering. 

In some elastics the variety of the pebbles represented is relatively 
simple (oligomictic) , whereas others contain a complicated and diver- 
sified pebble suite (polymictic) . Pebble composition is a useful attribute 
for decipherment of the origin of the deposit and for interpreting condi- 
tions under which the deposit accumulated. 

Limonite, calcite, silica, clay, and a combination of two or more of 
tjiese minerals are common bindents. 



' Van Tuyl, F. M., Profesfor and Head of Geology Department, Colorado School of Mines, Golden, 
Colorado, 

- Shrock, R. R. Associate Professor of Geology, Massachusetts Institute of Technology, Cambridge, 
Massachusetts. 



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Comments on Sedimentary Rocks 



73 



Nature of sediments 



Angular particles more 
than 2 mm. in greatest 
dimension 



Rounded particles more 
than 2 mm. in greatest 
dimension 



Sedimentary rocks 



Rubble composed of sharpstones 



Gravel composed of roundstones 



Sharpstone 



Roundstone 



CONGLOMERATE 



Angular and rounded par- 
ticles of rocks and min- 
erals ranging in greatest 
dimension from 2 mm. 
to 0.06 mm. 



Volcanic fragments = Tuff 

Mixture of rock and mineral frag- 
ments 

Quartz + Feldspar 

Quartz + other minerals in large 
amount 

Quartz + other minerals in small 
amount 



Tuffstone 
Graywacke 



.^rkose 
Normal 



Quartzose 



SANDSTONE 



Rock and mineral parti- 
cles ranging in greatest 
dimension from0.06 mm. 
to 0.001 mm. and col- 
loidal particles les-s than 
0.001 mm. in greatest 
dimension 

Fe^ and Fe™ compounds 
precipitated inorgani- 
cally and organically as 
concretions, nodules and 
layers 

Impurities commonly 
present in the layers 



Volcanic ash 

Silt particles - 0.06 to 0.001 mm. 
Clay materials less than 0.01 mm. 
Silt -I- Clay -|- Water = Mud 



Ashstone 
Siltstone 
Claystone 
Mudstone 



Iron concretions 

Iron compounds -|- mud, silica, 
etc. 



Concretionary 
Precipitated 



IRONSTONE 



Siliceous inorganic frag- 
ments less than 0.06 mm. 
in greatest dimension 

Siliceous organic hard 
parts and their frag- 
ments 

Silica precipitated as 
odiites, pisolites, etc. 

Silica precipitated from 
suspensions and solu- 
tions 



Inorganic fragments 

Diatom frustules, radiolarian 
skeletons and sponge spicules 

Siliceous concretions 

Chert, flint, sinter, etc. 



Fragmental 

Concretionary 
Precipitated 



8ILICA8T0NE 



Plant structures — spores, 

fronds, leaves, wood, etc. 

Inorganic sediment 

Waxes, resins, etc., from 

decomposition of plants 



Plant debris; inorganic impurities 
Plant fluids 



Coal 



Calcite and Aragonite fragments 

Calcareous organic hard parts — shells, exoskeletons, plates, 
spines, and fragments 

Organically and inorganically precipitated concretions 

Inorganically precipitated CaCOi — Evaporation, etc. 

Organically precipitated CaCOj — (1) by NHi from decom- 
position; (2) loss of CO2 to plants; etc. 



Fragmental 

Concretionary 

Precipitated 



Dolomite fragments 
Dolomitized organic hard parta 
Dolomitic concretions 
Inorganically precipitated dolomite 
Organically precipitated dolomite 



Fragmental 

Concretionary 

Precipitated 



LIMESTONE 



DOLO8T0NE 



O be 



Fragments of anhydrite, gypsum, halite, alkali, nitrate cali- 
che, etc. 



Fragmental 



Evaporites — minerals 
precipitated du'ing 
evaporation of saline 
waters 



Anhydrite 
Gypsum 
chlorides 
Nitrates 
Other rare salts 



Precipitated 



Anhydrock 
Gyprock 



sauna- 

8TONB 



>>T3 

•r; 0) 



S-3 

o V 

Q a 



Figure 34. Classifiication of sedimentary rocks. (From Shrock, 
Sequence of Layered Rocks.) 



74 Subsurface Geologic Methods 

The thickness of conglomerates and breccias ranges from inches to 
thousands of feet. Extreme variations may occur in relatively short dis- 
tances horizontally. These rocks may grade in all directions into time 
equivalent though lithologically dissimilar strata. They frequently rep- 
resent the basal phase of many formations and thus serve as criteria for 
recognizing unconformable relationships. Intraformational, coarse elas- 
tics have also been observed in many major lithologic units. 

A systematic analysis of the coarse elastics should include such studies 
as (1) type of pebbles, (2) ratio of pebble types, (3) degree of sorting, 
(4) fabric patterns, (5) alteration of pebbles, (6) type and distribution 
of bindents, (7) thickness trends and variations, (8) shape of pebbles 
(roundness, sphericity, flatness), and (9) relationship to adjacent de- 
posits. The results of these investigations should be recorded whenever 
possible in graphic form (histograms, percentage curves, composition 
triangles, and fabric diagrams) . 

Sandstones 

Sands and sandstones represent the medium-grained clastic sediments 
and are composed of grains of various rocks and minerals ranging from 
1/16 to 2 millimeters in diameter. 

Lithologists frequently assume sandstones to be composed primarily 
of quartz. This concept should be discouraged, as detailed examinations 
reveal the composition of sandstones to be extremely diverse and complex 
as exemplified by Pettijohn:^ 

Rough inspection of 50 thin sections of sandstone chosen at random from 
the University of Chicago collection shows than 45 percent are graywackes 
and subgraywackes, 35 percent orthoquartzites, and 20 percent are arkoses. 

A graywacke, according to Pettijohn, is composed of large very 
angular grains, mainly quartz, feldspar, and rock fragments (chiefly chert, 
phyllite, and slate). The grains are set in a prominent-to-predominant 
"clay" matrix which was, on low-grade metamorphism, converted to a mix- 
ture of chlorite and sericite and partially replaced by carbonate. 

An orthoquartzite is a sedimentary quartzite developed as a result 
of excessive silicification without the impress of metamorphism. 

An arkose or arkosic sandstone, according to the Committee on Sedi- 
mentation, contains 25 percent or more of feldspar derived from the dis- 
integration of acid igneous rock of granitoid texture. 

The terms "graywacke" and "arkose" have recently received con- 
siderable attention in studies of the relationship between tectonism and 
sedimentation. Arkoses are considered to be typically developed in in- 
tracratonic basins, whereas graywackes accumulate dominantly within 
geosynclinal downwarps. The contrasting features of these two rock types 
are given by Pettijohn:'* 



' Pettijohn, F. J., Sedimentary Rocks, p. 229, New York, Harper and Bros. 1949. 
' Pettijohn, F. J., op. cit., p. 261. 



Comments on Sedimentary Rocks 75 

Arkose usually is coarser-grained, lighter in color (pink or light gray). 
It may be strongly cross-bedded, is cleaner sorted, i.e. is without interstitial clay 
or silt and consequently, is bound by introduced mineral cement (usually car- 
bonate). Arkose is closely associated with coarse conglomerates that contain 
much granite debris, and it is usually terrestrial in origin. Graywacke is the 
opposite in nearly all particulars. It is finer-grained, dark in color, rarely cross- 
bedded, but with marked graded bedding if any bedding is visible, with an 
interstitial paste having the composition of a slate, and hence without mineral 
cement. Graywacke is interbedded with black, pyrite slates and associated with 
pillow lavas and chert. It is usually marine. 

Sandstones grade in texture from very coarse (2 to 1 mm.) to very 
fine (i to yg mm-)- Sorting may be in certain instances exceptionally 
high, whereas in others it is extremely low. The coefficient of sorting 
may be determined by construction of a cumulative-frequency curve. 

The colors of sandstones have a wide range (white, gray, red, green, 
to black) depending on composition of the grains, type and amount of 
cement, and degree of weathering. Cementing materials commonly con- 
sist of limonite, hematite, carbonate silica, organic material, anhydrite 
and gypsum, and clay. Pure quartz sandstones are indicative of stable 
shelf-depositional conditions. 

Bedding characteristics of sandstones may be massive to thinly lamin- 
ated. Bedding surfaces may be sharp to gradational, parallel or con- 
verging. 

Accessory components comprising the heavy-mineral fraction of 
arenaceous rocks consist of such minerals as pyrite, hornblende, pyroxene, 
olivine, magnetite, leucoxene, garnet, tourmaline, zircon, mica, staurolite, 
anatase, and glauconite. 

Sandstone varieties should be designated by a qualifying adjective 
whenever possible; i.e., glauconitic, micaceous, garnetiferous, pyritic, and 
hornblendic. 

Studies of arenaceous rock types should treat: textural aspect, 
grain fabric, type and distribution of cement, character of grains (sur- 
face features, alteration, shape, inclusions), grain composition, and var- 
iations in porosity and permeability. These values may be determined by 
thin sections, screen analysis, heavy-liquid separation, polished surfaces, 
and chemical analysis. In subsurface investigations fluid and gas con- 
tents may be evaluated by core analysis and by data obtained from var- 
ious well-logging methods. 

SiltstoTie 

Siltstones, the indurated equivalent of silts, are fine-grained elastics 
represented by particles ranging in size from 1/16 to 1/256 millimeter. 
These rocks vary considerably in color and structure. They frequently 
contain various compounds of organic matter and as a rule yield normal 
suites of heavy accessory minerals. In many instances rocks of this cate- 
gory are classified as sandy (arenaceous) or silty shales. 



76 Subsurface Geologic Methods 

Shale and Mudstone 

Shales and mudstones constitute the finest of the clastic materials. 
Particle sizes range below 1/256 millimeter. The primary constituents of 
these rocks are represented by the complex clay minerals which are ex- 
tremely difficult to determine, especially without reverting to advanced 
petrographic procedure. Silica is the dominant element in shales and 
mudstones. It is present either as free silica (quartz) or in the form of 
silicates. Alumina is next in importance to silica. Other elements in- 
clude titanium, iron, manganese, calcium, sodium, potassium, and phos- 
phorous. The common clay minerals include kaolinite, montmorillonite, 
and illite. 

Many lithologists arbitrarily designate all argillaceous elastics as 
shales. This terminology should be applied to only those rocks exhibiting 
fissility or lamination. Rocks failing to show these structural features 
should be termed mudstone or claystone depending on the plasticity value. 

The porosity of argillaceous rock types may range up to 50 percent, 
although the average value, according to Pettijohn, is 13 percent. 

Hybrid types of argillaceous rocks include marlstones (50 to 80 per- 
cent carbonate), clay ironstone (rich in siderite), porcellanite (high in 
opaline silica), and black shale (exceptionally rich in disseminated or- 
ganic matter) . 

Important accessory minerals in shales and mudstones include mica, 
glauconite, pyrite, silt, and sand. Cementing materials may be of a silic- 
eous, ferruginous, calcareous, carbonaceous, or bituminous nature. 

In descriptive work these sediments should be qualified by proper 
adjectives; i.e., glauconitic, carbonaceous, dark gray shale. 

During the past few years considerable attention has been given to 
clay mineralogy. This study involves such methods as elutriation, thin- 
section, rate of settling. X-ray, specific gravity, spectroscopy, differential 
thermal analysis, electron microscope, staining and chemical analysis. 

Limestone and Dolostone 

Limestones and dolostones may be of clastic or chemical origin or 
may be developed as a result of both processes of deposition. The chem- 
ical composition of these rock types varies considerably. Rarely does 
there occur a pure limestone, CaCOs, or dolomite, CaMg(C03)2, because 
of the inclusion of detrital materials. These two carbonate types invari- 
ably grade into each other. In certain instances limestones and dolomites 
contain an abundance of organic remains, whereas in other cases the re- 
mains of organisms are absent or nearly so. The dominant organic re- 
mains within carbonate rocks include those of algae, mollusks, corals, 
echinoids, Bryozoa, ostracodes, and Foraminifera. 

The partial or total alteration and replacement (metasomatism) of 
limestone strata have been recognized and have resulted in dolomitization, 
chertification, and phosphatization. Diagenetic changes frequently oblit- 



Comments on Sedimentary Rocks 77 

erate original textures and structures and develop new ones. Carbonates 
accumulate under various environmental conditions: stable shelf, mildly 
unstable shelf, intracratonic basin, and geosyncline. 

The color of carbonate rocks varies from white to black; crystallinity 
ranges from fine to coarse. Limestones and dolostones may be thick- 
bedded to finely laminated, vuggulated, oolitic, or pisolitic. Foreign con- 
stituents as shown by insoluble-residue work include chert, clay, quartz, 
pyrite, glauconite, arenaceous Foraminifera, and silicified fragments. 

During the past several years, oil companies have shown intense in- 
terest in reef limestones and dolostones as possessing great oil and gas 
potentials. Systematic surveys of all possible reef areas are being made 
and criteria are being collected to evaluate these deposits more accurately. 

The following techniques have been followed in the study of car- 
bonate rocks: thin-section, polished surface, insoluble residue, chemical- 
stain test, spectrochemical, porosity and permeability, relative solubility, 
petrofabrics, and chemical analysis. In addition to these methods, elec- 
trical, radioactive, and micrologging have added considerably to our 
knowledge of subsurface carbonate characteristics. Detailed carbonate in- 
vestigations contribute to improvement of more efficient production and 
well-completion methods. 

Evaporites 

The sediments composing the evaporites are primary precipitates 
which have resulted for the most part from the evaporation of saline solu- 
tions. The best-developed evaporite sections in North America are found 
in the Permian Basin of west Texas and eastern New Mexico, and in the 
Silurian salt basins of Michigan and New York. The evaporites are rep- 
resented by the sulphates (anhydrite, gypsum), chlorides ( mainly halite), 
and minor carbonates. The sodium and potassium sulphates and the 
nitrates are also included in the evaporites. Rock salt, anhydrite, and 
gypsum are the most commonly encountered. 

Rock-salt beds, ranging from inches to 80 feet in thickness, are gen- 
erally associated with anhydrite and gypsum. They are frequent in red- 
bed sections. Their purity, color, and texture are variable. Crystallinity 
of the salt varies from fine to coarse. Great masses of this material have 
flowed and produced salt domes in the Gulf Coast area of Texas and 
Louisiana and in Germany, Iran, and Russia. 

Anhydrite and gypsum occur as bedded deposits mainly in nonmarine 
stratal sequences and are commonly associated with dolostone and shale 
phases. Frequently these rocks exhibit normal lamination or corrugated 
lamination. Anhydrite generally ranges in color from white to dark gray 
and assumes a fine to coarse crystallinity. The structural and textural 
features are sometimes appreciably modified by conversion to gypsum 
upon hydration. Gypsum frequently occurs in argillaceous strata as trans- 
parent crystals of selenite. 



78 Subsurface Geologic Methods 

Anhydrite and gypsum beds frequently serve as dependable correla- 
tion markers in red-bed sections. In certain instances they are quite con- 
tinuous and uniform. The Blaine anhydrite in the Permian section of 
southeastern Colorado is an example. 

The study of evaporite rocks should include chemical analysis; tex- 
tural, structural, and spectrophemical investigations; thin-section analysis; 
and rhythmic characteristics. 

Carbonaceous Rocks 

Carbonaceous rocks are represented by three types of residue : humus, 
peat, and sapropel. Humus is produced within the upper part of the soil 
phase. Peat originates from partial decay of plant material under fresh- 
water swamp conditions. Sapropel (high in fatty and protein substances) 
results from concentration of complex organic compounds which accumu- 
late on the bottoms of lakes, lagoons, and quiet-water embayments. 

Coal is by far the most common variety of carbonaceous deposit and 
is classified according to degree of coalification and physical character- 
istics. Lignite is the lowest grade and anthracite the highest. Some of the 
more common ingredients of coal include vitrain, fusain, clarian, and 
durain. The chemical composition of coal is extremely variable. The main 
constituents in coal are carbon, hydrogen and oxygen, nitrogen, sulphur, 
and water. 

Sections containing coal beds generally present many complex prob- 
lems to the stratigrapher and structural geologist owing to their extreme 
diversity of lithologies and depositional irregularities. 

Considerable attention in recent years has been given to cyclothemic 
sequences in coal sections. In the Midcontinent region, in the Illinois 
basin, and in Kansas, coal cyclothems and megacyclothems have been 
used advantageously in establishing correlations and decipering structure. 

The investigation of coals includes such analyses as thin section, 
moisture, volatile matter, fixed carbon, and ash. 

Sapropelic deposits have been considered by some geologists to be 
responsible for the formation of petroleum; however, there is no agree- 
ment as to the physico-chemical processes of convfersion. 

To evaluate sapropelic rocks properly, studies involving thin-section, 
heat analysis, and ether solubility should be made. 

Miscellaneous Rock Types 

1. Siliceous rocks: The most common siliceous rocks are repre- 
sented by the microcrystalline cherts and flints. Colors are extremely 
variable : white, green, red, brown, and black. Chert and flint are common 
constituents in the carbonate rocks and occurs as lenses, nodules, thin 
beds, and fracture fillings. The saturation of argillaceous strata with 
opaline silica produces siliceous shales and porcellanites. The presence of 
abundant diatoms and volcanic ash produces this type of deposits. Cher- 



Comments on Sedimentary Rocks 79 

tified layers in carbonate sections have served many times as dependable 
correlation criteria. 

2. Ferruginous rocks: The ferruginous sediments may be classified 
as carbonates (siderite), iron silicates (glauconite), ferric oxides and 
hydroxides (hematite, limonite), and sulphides (pyrite, marcasite) . 

Iron carbonate (siderite) is commonly associated with argillaceous 
cherty strata in the form of beds, lentils, and concretions. Siderite also 
occurs in the carbonate rocks (limestones, dolostones) . 

Chamosite is the principle iron-bearing silicate in sedimentary rocks, 
principally the finer-grained elastics. Glauconite constitutes an important 
silicate in many types of marine strata (shales, sandstones, and lime- 
stones) . This mineral should be recorded in all stratigraphic investiga- 
tions because of its correlation value and depositional environment index. 
Glauconite varies in color from pale green to greenish-black. It occurs 
mainly as rounded to elliptically shaped granules commonly exhibiting 
shrinkage fractures. 

Limonite and hematite are common oxide bindents of sedimentary 
elastics. These minerals occur as disseminations or in some cases as oolites. 
In the Silurian strata of the Appalachian region sedimentary hematites 
are commercially important. 

Pyrite and marcasite occur in all types of sedimentary rocks in 
varying percentages. They may be present as nodules, crystal aggregates, 
and disseminations. Pyrite is particularly common in highly organic, 
black shales. Occasional beds of pyrite have been observed. 

3. Manganiferous rocks: Manganese occurs in minor amounts in all 
sedimentary rocks. The oxides, hydroxides, and carbonates are the chief 
mineral types. 

4. Phosphatic rocks: Phosphate-bearing rocks are commonly re- 
ferred to as phosphorites. Phosphatic materials are found primarily in 
shales and limestones, and may be either of primary or secondary origin, 
or both. Color of phosphates varies from brown to black, although by 
leaching it may assume lighter hues. The material may be bedded or may 
occur as concentrically banded oolites and nodules. Nodules range up 
to several centimeters in diameter. Shell fragments are frequently phos- 
phatized. The origin of phosphate deposits appears to be related to animal 
remains (bones, guana) . 

Texture of Sedimentary Rocks 

The texture of a rock refers to the size, shape, and arrangement of 
the individual particles. Textural values are extremely important in the 
description of a rock. The size of clastic particles may be expressed by 
the terms coarse (gravel), medium (sand), and fine (clay) depending on 
their dimensions as determined by selective screening or by actual meas- 
urement. The limits of various grain sizes are more or less arbitrary. 



80 Subsurface Geologic Methods 

Wentworth's subdivision of grade size ^ has been widely accepted by most 
sedimentologists. Particle-size percentages may be graphically represented 
by histograms and frequency curves, and such values as median, coeflBc- 
ient of sorting, skewness, and kurtosis determined. These values permit a 
quantitative representation of particle size. 

The shape (sphericity, roundness, and flatness) of particles may be 
determined by various methods and numerical values given. Such terms 
as "angular" (showing very little or no abrasion) , "subangular" (show- 
ing some effect of wear), "subrounded" (showing considerable abrasion), 
and "rounded" (exhibiting conspicuous wear) are commonly used to 
designate degree of angulation. Some medium-grained elastics exhibit 
pronounced uniformity in grain angularity, whereas others show consid- 
erable variation of angularity. 

Close examination of the elastics reveals in certain instances distinct 
fabric pattern of the grains. Petrofabric diagrams (figs. 69 and 70) are 
helpful for illustrating these orientation trends. 

In addition to the size, shape, and arrangement of grains, special at- 
tention should be given the characteristics of grain surfaces, such as degree 
of polish, smoothness, striation, and pitting. These features have definite 
genetic significance. 

Permeability and porosity are controlled in large part by the texture 
of the rock. Since these two factors are of primary importance to the oil 
geologist in production problems, textural attributes of producing strata 
should be carefully evaluated. 

Textures of carbonate and evaporite rocks range from fine to coarse 
crystalline. Textures of chemical sediments are clearly outlined by Petti- 
john.^ Such terms as "macrocrystalline" (granoblastic, over 0.75 mm.), 
"mesocrystalline" (porphyroblastic, 0.20 to 0.75 mm.), "microcrystalline" 
(0.01 to 0.20 mm.), and "cryptocrystalline" (less than 0.01 mm.) are 
discussed. 

According to DeFord,'^ "the grade scales for clastic rock are not 
suitable for carbonate rock, because the names of the scale units imply 
the clastic origin of the rock." DeFord recommends the terms and size 
limits given in figure 35. 

Structures in Sedimentary Rocks 

Structures developed in and exhibited by sedimentary strata are 
numerous and varied. Special attention should be given these features, 
because of their usefullness in deciphering depositional environments, 
stratigraphic succession, and structural anatomy. Two types of structures 
in sedimentary rocks are recognized: inorganic and organic. The former 
includes such features as ripple marks, swash marks, current marks, pit- 

° Wentworth, C. K., Fundamental Limits to the Sizes of Clastic Grains: Science, vol, 77. pp. 633 634, 
1933. 

^ Pettijohn. F. J., op. cit., p. 73. 

' DeFord, R. K., Grain She in Carbonate Rock: Am. Assoc. Petroleum Geologists Bull., vol. 8, pp. 
1921-1926, 1946. 



Comments on Sedimentary Rocks 



81 



and-mound, rain-drop impressions, gas pits, crystal imprints, mud cracks, 
bedding, cross-lamination, scour and fill, imbrication, laminar corruga- 
tion, intra- and interstitial flow, unconformities and diastems, nodules, 
geodes, septaria, stylolites, cone-in-cone, and veinlets. Organic structures 
are represented by tracks and trails, borings, and petrifactions. 

Sedimentary structures and their significance are treated in full by 
Petti John ^ and by Shrock.^ 



X? 

Q.I;; 



02 

— o 

z- 

xs 

OL « 
<5 



RADIX 10 



MEDIUM 

MEGAGRAINED 3.2 



MEDIUM 

MESOGRAINED .32 



MEDIUM 
PAUROGRAINED -032 



MEDIUM 
MICROGRAINED .0032 



CRYPTOGRAINED 



1/2 - 
1/4 - 
l/S - 
k/16 - 
1/32 - 
1/64 - 
1/128 - 
1/256 - 
1/512 - 
1/1024- 



RADIX 2 



Figure 35. Tentative scale for carbonate rocks, grain diameters in millimeters, plotted 
logarithmically. Scale based on radix 2 (used by Wentworth) added for com- 
parison. (From DeFord. Reproduced permission Am. Assoc. Petroleum Geolo- 
gists.) 



' Pettijohn, F. J., op. cit., pp. 120-168. 

' Shrock, R. R., Sequence in' Layered Rocks, New York, McGraw-Hill Book Co., Inc., 1948. 



82 Subsurface Geologic Methods 

Color of Sedimentary Rocks 

The color of sedimentary rocks is controlled by the grain size, by the 
composition of the grain, and by the chemical pigmentation. Colors may 
be primary or secondary, or both. 

Several proposals have been made in order to standardize colors of 
sedimentary rocks more adequately. DeFord's statement,^^ "The descrip- 
tion by geologists of the colors of rock outcrop and rock cuttings from 
test wells is completely anarchistic: every man for himself," should be 
carefully considered and recognized as being the truth. What one individ- 
ual designates as "brick-red" another might consider "orange-red." The 
terms "tan" and "buff" are frequently used. Tan, to one person, may be 
a medium-brown to another. 

In order to improve standardization of rock colors among geologists, 
it is recommended that the rock-color chart prepared by the Rock-Color 
Chart Committee and published by the National Research Council in 1948 
be carefully followed. 

Evaporites, carbonates, and some argillaceous sediments, which range 
from white to light gray, indicate the total or nearly total absence of 
bituminous and carbonaceous impurities. The dark coloring (dark gray 
to black) in rocks is invariably due to the presence of organic matter, 
black iron sulphides, manganiferous constituents, or dark detrital min- 
erals. Upon weathering, these dark hues may become lighter as a result of 
leaching. Sediments derived from basic igneous rocks assume dark colora- 
tion. 

Iron compounds (limonite, hematite) produce yellow, tan, and red 
hues. The presence of red feldspar in arkoses are largely responsible for 
reddish and pinkish colorations. 

Greenalite, glauconite, epidote, olivine, chlorite, and ferrous iron 
compounds are responsible for greenish colors. 

Special notation should be made during the recording of colors as 
to whether the sediment at the time of recording is wet or dry. Rocks 
when wet invariably assume darker colors. Mention should also be made 
as to whether a color represents a weathered or unweathered surface. The 
latter case is clearly exemplified by the Apishapa shale (Upper Cretace- 
ous) of Colorado. In outcrop this member assumes a pale orange to a 
light-buff color. In the subsurface or on fresh exposure it is gray-black. 
Many similar examples may be cited. 

DiAGENESIS of SEDIMENTARY RoCKS 

Following the deposition of a sediment, certain physical and chemical 
processes are initiated which tend to adjust the sediment to its environ- 
ment. These processes are varied and complex, and their relationships 
are unknown in many instances. Certain modifications of the sediments 



" DeFord, R. K., Rock Colors: Am. Assoc. Petroleum Geologists Bull., vol. 28, no. 1, pp. 128-137, 
Jan. 1944. 



Comments on Sedimentary Rocks 83 

occur prior to lithification, whereas others are not evident until long after 
burial. 

Compaction, cementation, and metasomatism are largely responsible 
for the rearrangement and replacement of sedimentary constituents. 

Compaction is most obvious in the fine-grained elastics (shale, mud- 
stone, claystone) . Porosity is substantially minimized and original fabric 
drastically modified. New minerals may be formed by closer packing and 
by increased pressures and temperatures produced as a result of weight 
of overburden. Compaction generally has little or no diagenetic effect on 
the coarse elastics. 

Cementation is responsible for many modifications of a sediment. 
Porosity and permeability are reduced, new minerals formed or the or- 
iginal minerals replaced in whole or in part. Secondary silica deposited 
around quartz grains and giving rise to secondary facets is a common 
phenomenon. 

Metasomatism, involving replacement and alteration, is a common 
process in sedimentary rocks, particularly in the carbonates. Calcite is fre- 
quently replaced by dolomite and silica. Several stages of replacement 
may be involved. 

Questions 

1. In evaluating a conglomerate, what features should be considered? 

2. What is the difference between a graywacke, an orthoquartzite, and 
an arkose? 

3. Define shale and mudstone. 

4. What methods of study may be followed in analyzing a shale or mud- 
stone? Limestone or dolostone? 

5. What are the most common evaporites? 

6. State the difference between humus, peat, and sapropel. 

7. What is porcellanite, glauconite, and chert? 

8. Define texture. 

9. What is a petrofabric diagram? 

10. Give an example of a metasomatic change in sedimentary rocks. 



CHAPTER 4 

SUBSURFACE LABORATORY METHODS 

MICROPALEONTOLOGIC ANALYSIS 

L. W. LeROY 

Prior to 1925, micropaleontology played an insignificant role in 
stratigraphic and paleontologic investigations. It was not until that year 
that the science was recognized and appreciated as a valuable tool in 
surface and subsurface problems of the petroleum industry. All major 
and many minor oil companies now sponsor micropaleontologic labora- 
tories. 

The economic micropaleontologist is essentially a microstratigrapher. 
His time is not only devoted to the paleontologic aspects of strata, but 
also to lithology, to detrital mineralogy, and to the many other techniques 
which aid in the solution of stratigraphic problems. 

Methods followed by micropaleontologists are extremely variable 
and are controlled by the type of problem (surface or subsurface), the 
time allocated to the problem, the quality of personnel involved, and 
company policy. In some areas only major faunal divisions of sections 
are desired, whereas in other areas it is necessary to introduce detailed in- 
vestigations before the problem under consideration can be properly 
solved. The micropaleontologist should be familiar with the field geolo- 
gist's assignment and should visit field operations whenever it is deemed 
necessary in order to coordinate the laboratory work properly. He should 
systematize laboratory routine so that data may be obtained as soon as 
possible for final analysis. It should be remembered that most micro- 
paleontological problems are of the "pressure" variety, and that not un- 
commonly management desires results before the project is started. The 
micropaleontologist must be versatile, having a knowledge of all varieties 
of microfaunas as well as being able to evaluate their significance and rec- 
ognize their use limitations. 

Micropaleontology has aided materially in evaluating unconformi- 
ties, structural conditions, and facies changes, in dating and correlating 
strata, and in interpreting depositional environments. The science has its 
limitations, and this fact should be recognized; otherwise, incompetent con- 
clusions and interpretations may be introduced. Micropaleontologic data 
should be coordinated with all other available stratigraphic information. 

Only those microfossils that have been used by paleontologists in 
the oil industry are discussed here briefly. Some types are more applic- 
able in the solution of stratigraphic problems than are others. Those 
herein considered are Foraminifera, ostracodes, Radiolaria, conodonts, 
otoliths, fish scales, calcareous algae, diatoms, spores and pollen, and 
grass seeds. 









Figure 36. Assemblages of Foraminifera from the Late Tertiary of the South Pacific region. 
These single-celled micro-organisms have had world-wide use in correlating strata (X 15). 



86 Subsurface Geologic Methods 

FORAMINIFERA 

The Foraminifera (fig. 36) are single-celled, microscopic animals be- 
longing to the phylum Protozoa. These forms, ranging in diameter from 
0.01 mm. to 50.0 mm., attain their best development in marine environ- 
ments, although some genera and species occur profusely in brackish- and 
even fresh-water habitats. Their tests (shells) are extremely variable in 
structure and composition. Certain genera secrete chitinous, arenaceous, 
and siliceous tests, although most of them produce calcareous structures. 
Tests of the Early Paleozoic Foraminifera are structurally simple; those 
of the Mesozoic and Cenozoic exhibit more complexity and variety. Fora- 
minifera occur abundantly locally in the Late Paleozoic deposits and even 

EXPLANATION OF PLATE 1 

Figure 
1- 3. Cibicides telisaensis LeRoy X 82. Fig. 1, ventral view. Fig. 2, dorsal view. 

Fig. 3, peripheral view. 
4- 6. Anomalina sp. A LeRoy X 75. Figs. 4, 5 opposite sides. Fig. 6, peripheral 

view. 
7- 9. Cibicides dorsopustulosus LeRoy X 43. Fig. 7, dorsal view. Fig. 8, ventral 

view. Fig: 9, peripheral view. 
10-12. Cibicides foxi LeRoy X 75. Fig. 10, dorsal view. Fig. 11, ventral view. Fig. 

12, peripheral view. 
13-15. Anomalina sp. A LeRoy X 65. Figs. 13, 15, opposite sides. Fig. 14, peripheral 

view. 
16-18. Cancris auriculas (Fichtel and Moll) X 47. Fig. 16, dorsal view. Fig. 17, 

ventral view. Fig. 18, peripheral view. 
19-21. Valvulineria aff. inaequalis (d'Orbigny) X 75. Fig. 19, dorsal view. Fig. 20, 

ventral view. Fig. 21, peripheral view. 
22-24. Eponides praecintus (Karrer) X 35. Fig. 22, dorsal view. Fig. 23, ventral 

view. Fig. 24, peripheral view. 
25-27. Quinqueloculina sp. H LeRoy X 29. Figs. 25, 26, opposite sides. Fig. 27, 

apertural view. 
28-30. Valvulineria araucana (d'Orbigny) car. malagaensis Kleinpell X 47. Fig. 28, 

dorsal view. Fig. 29, ventral view. Fig. 30, peripheral view. 
31-33. Baggina inflata LeRoy X 47. Fig. 31, dorsal view. Fig. 32, ventral view. 

Fig. 33, peripheral view. 
34-36. Globorotalia barissanensis LeRoy X 73. Fig. 34, peripheral view. Fig. 35, 

dorsal view. Fig. 36, ventral view. 
37, 38. GlobigerineUa aequilateralis (Brady) X 48. Fig. 37, side view. Fig. 38, per- 
ipheral view. 
39, 40. Globigerina siakensis LeRoy X 54. Fig. 39, dorsal view. Fig. 40, ventral view. 

41,42. Globigerinoides trilocularis (d'Orbigny) X 37. Fig. 41, ventral view. Fig. 42, 

dorsal view. 
43, 44. Globigerina baroemoenensls LeRoy X 41. Fig. 43, dorsal view. Fig. 44, ventral 

view. 
45,46. Globigerinoides sacculiferus (Brady) var. irregularus LeRoy, n. var. X 39. 

Opposite views. 

more so in strata of Mesozoic and Cenozoic age. It is not uncommon to 
find limestones and marlstones of the Pennsylvanian and Permian com- 
posed almost entirely of the remains of these organisms (fusulinids) . 
Many Cretaceous chalks and Eocene limestones contain multitudes of 
foraminiferal tests as well as numerous argillaceous deposits of the Late 
Tertiary (fig. 36), which accumulated under tropical or subtropical con- 
ditions. 



Subsurface Geologic Methods 




Drawn by A. Hamld 

Plate 1. Tests (shells) of the small Foraminifera. (See p. 86 for explanation.) 



Subsurface Geologic Methods 




■jifmM >j , 'tgf te, 



Plate 2. The large foraminifer Cyclocypeus from the East Indies. Thin section 
shown in lower microphotograph. Top six views 8x; bottom view 18x. (From 
Tan. Wet. Meded.) 



Subsurface Geologic Methods 




Plate 3. Thin sections of a large foraminifer (Lepidocyclina) 30x. The in- 
ternal structure of these forms must be studied before identification 
mav be established. (From Schefien. Wet. Meded.) 



Subsurface Laboratory Methods 87 

According to Cushman ^ "The habits and physiologic characters of 
the animal and its relationships to the environment are very incompletely 
known." The Foraminifera reproduce both sexually (resulting in micro- 
spheric varieties) and asexually (resulting in megalospheric varieties). 
The tests of these two generations vary considerably in size and structure 
and must be considered in speciation studies. Some forms are bottom- 
dwelling (benthonic), some are floating (pelagic) types, whereas others 
prefer attachment (sissile) . 

Benthonic assemblages adjust to temperature. At any given locale, 
shallow-water suites may vary radically from their deeper-water neigh- 
bors. Shallow-water forms (genera and species) of tropical environs are 
conspicuously divergent from shallow-water assemblages of higher lati- 
tudes. These variables must be considered in correlation interpretations, 
as two assemblages possessing identical time values may be quite dis- 
similar in composition. Pelagic suites, more restricted in genera and 
species than benthonic assemblages, off^er the best possibilities for long- 
range correlations because of their nondependency on bottom ecology and 
the temperature-depth factor. 

The tests of Foraminifera and other micro-organisms possessing hard 
parts are released from sediments by various means. A procedure com- 
monly applied is first to examine the rock sample carefully and record 
any pertinent information such as lithology, color, minerals, and cementa- 
tions, which may assist in evaluating the ecology of the contained fauna. 
If the sediment is not indurated, it is crushed to ±^-inch fragments, 
soaked in water (boiled if necessary) for several hours, and then washed 
through a series of screens (80-, 100-, 150-mesh) in order to remove the 
fine elastics. The washed residue may then be examined either under water 
or dried by means of a binocular microscope (X 30 to X 60) . If the ratio 
of the microfauna to the detrital material is not large, the assemblage may 
be concentrated by heavy liquids or by gravity methods. Swirling the 
material under water in an examination dish frequently aids in segrega- 
tion. In the event the host sediment is highly cemented (calcareous, silic- 
eous, or ferruginous) or compacted, the tests may be released by excessive 
cracking of the material. If this method is not feasible, thin-section or 
polished-surface studies are required. 

When an assemblage is once released and concentrated, the laborious 
procedure of examination and recording follows. In the initial stages of 
micropaleontologic work, it is essential that all genera, species, and species 
varieties be accurately determined and their relative abundances tabulated. 
It is on the basis of these data that distribution charts (figs. 37, 38, and 
39) are prepared, faunal zones defined, sections subdivided, index species 
determined, and correlations established. 

Foraminiferal correlations are based on (I) single species or genus, 

^ Cushman, J. A., Foraminifera, Their Classification and Economic Use: p. 3, Cambridge, Mass., Har- 
vard University Press, 1948. 



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THE MAXIMUM THICKNESS OF A SYMBOL IN 
EACH COLUMN REPRESENTS GRAPHICALLY THE 
OPTIMUM DEVELOPMENT OF AN ASSEMBLAGE 
CORRESPONDING TO ITS S T R A T IG R A PHIC AL POS- 
ITION. A DECREASING THICKNESS INDICATES A 
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Figure 38. A foraminifera] distribution chart. Species 
left to right. Symbols represent relative abundance 
graphic positions. Such charts are essential for subd 
sections into paleontological zones. 



are recorded across the top from 
of each species at various strati- 
ividing homogeneous marine shale 



90 



Subsurface Geologic Methods 



FEET 



BED 



285 



270- 



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240 



225 



210 



195 



180- - 



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Pseudopo ly mo rph i r 



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Pseudopolymorph ino 




Figure 39. Unique manner of showing index microfossils in their stratigraphic posi- 
tion. (From Lalicker. Univ. Kansas Paleontological Contrib.) 



Subsurface Labobatoey Methods 91 

(2) one or more species or genera, ^3j general assemblages, (4) relative 
abundance of one or more than one species or genus, (5) ratio of various 
species and genera, (6) evolutionary development, and (7) faunal 
sequence. 

The recognition of key species requires considerable detailed work. 
The index value of these forms must be repeatedly tested and compared in 
several stratigraphic sequences. For example, it required the writer two 
vears to determine ten key species from a 375-species fauna of the Middle 
Tertiary deposits of central Sumatra. A given species may have an index 
value in one area, whereas its value as such may be slight or even worth- 
less in adjacent areas. The responsibility to recognize and determine 
these peculiarities rests with the micropaleontologist 

It is general laboratory practice to develop a species type or control 
set for reference and comparison. Type species are either identified spe- 
cifically or are given work numbers. The latter procedure is most applica- 
ble when a reference is not available for establishing species determina- 
tions. 

In working foraminiferal faunas and interpreting their significance, 
it is necessary' constantly to keep in mind the facies concept. Natland ^ 
demonstrated that recent foraminiferal assemblages off the coast of 
southern California differ in composition from shallow to deep water. 
He recognized a similar faunal sequence vertically in the Pliocene section 
of the Ventura Basin, California. The significance of this relationship 
exemplifies that these various assemblages have transected time horizons, 
a characteristic that many faimas undoubtedly possess. This problem con- 
stantly confronts micropaleontologists when long-range correlations are 
based on benthonic assemblages. Unlike faunas may be of similar age, 
although similar faunas may not possess the same time value. 

LoA\Tnan ^ in his studies of the distribution of recent Foraminifera of 
the Gulf of Mexico has shown the extreme lateral variation of this group 
from fresh-water to brackish-water to open-sea en\"ironments. The object 
of this investigation was to improve "our ability to use fossil foraminifera 
as criteria of depositional environments, and, second, to help disentangle 
environmental and evolutionary factors in the three-dimensional dis- 
tribution of fossil faunas." 

A unique method employed by the Bataafsche Petroleum Maat- 
schappij (Royal Dutch-Shell) in preparing micropaleontologic facies logs 
is described by Ten Dam.- This method is based primarily on the quanti- 
tative occurrence of certain genera or groups of genera of Foraminifera. 
Ten Dam summarizes the general procedure as follows: 

The microfauna of each washed sample is divided into benthonic and 

' Natland, M. L., The Temperature and Depth Distribution of Some Recent and Fossil Foraminifera 
in the Southern California Region: Scripps Inst. Oceanography Boll., toL 3, no. 10. pp. 223-230. 1933. 

^ Lowman, S. W., Sedimentary Facies of Gulf Coast: Am. Assoc. Fetrolenm Geologists, voL 23, no. 
12, pp. 19391997, 1949. 

* Ten Dam, A.. Micro paleontological Fades-Logs: The Micropaleontologist, toL 1, no. 4, pp. 13-15, 
Oct. 1947. 



92 Subsurface Geologic Methods 

planktonic elements. The planktonic elements are the Globigerinidae and 
certain other genera. The rest of the microfauna will be almost entirely ben- 
thonic. The necessary micropaleontological data for compiling a facies-log are: 
(1) approximate number of the planktonic specimens; (2) approximate num- 
ber of benthonic specimens; (3) approximate number of the specimens of 
genera or species indicating more or less brackish waters; (4) approximate 
number of the specimens of the genera indicating deep water or cold water; 
and (5) approximate number of the specimens of large Foraminifera. 

De Sitter ^ and Ten Dam gave a key list of some ecologic conclusions 
based on facies logs. These are as follows: (1) A great number of plank- 
tonic specimens indicates a good connection with the open ocean; a grow- 
ing number of specimens indicates opening of the basin to the open sea; 
a diminishing number indicates closing of the basin. Absence of plankton 
may indicate very shallow water. (2) A large number of benthonic species 
is an indication of favorable living conditions. (3) A small number of 
benthonic species may indicate limited or unhealthy living conditions. 
(4) The development of very large foraminifera! faunas indicates shelf 
conditions and clear and shallow water. Combined with much plankton it 
may indicate submarine ridges. Without plankton it probably signifies 
nearness to the coast or location in a more or less closed basin. (5) A 
microfauna with many specimens of Cassidulina or other species that 
seem to imply deep water may be an indication of deep water, generally 
with favorable bottom conditions. 

None of these factors, according to Ten Dam, are in themselves de- 

EXPLANATION OF FIGURE 40 

Figure 40A. An assemblage from the Santa Barbara formation ("upper Pliocene") 
at Bath House Beach, city of Santa Barbara, Santa Barbara County, California. 
Note Cythereis pennata LeRoy (19) and C. kewi LeRoy (18). 

Figure 40B. A recent brackish-water assemblage from Sunset Lagoons, Orange 
County, California. Ammocytheridea sp. (21) is very closely related to Cytheridea 
beaconensis LeRoy. 

Figures 40C and 40D. A recent assemblage from the intertidal zone, Mussel Rock, 
Monterey Bay, California. Hemicythere palosensis LeRoy (23). 

Figures 40E and 40F. An assemblage from the Lomita facies ("Pleistocene"), Hill- 
top quarry, Palos Verdes Hills, Los Angeles County, California. 
4. Hemicythere? californiensis LeRoy. 

6. Bairdia verdesensis LeRoy. 

7. Cythereis glauca Skogsberg. 

16. Loxoconcha lenticulata LeRoy. 

17. Cytherelloidea californica LeRoy. 

18. Cythereis kewi LeRoy. 

19. Cythereis pennata LeRoy. 

20. Caudites fragilis LeRoy. 

21. Ammocytheridea sp. 

22. Ammocytheridea sp. 

23. Hemicythere palosensis LeRoy. 

cisive, but a combination of several factors may lead to a solution. Care- 
fully interpreted facies logs of a series of field and well sections may give 
some concept of lateral and vertical facies changes. 

° De Sitter, Geologie en Mijnbouv, new ser., vol. 3, no, P, pp. 225-237, 1941. 




Figure 40. Assemblages of marine ostrocodes (magnification ± 15). These organisms occur most 
profusely in shallow-water environments. They also occur abundantly in fresh-and brackish- 
water habitats. In many areas they serve as index fossils. (For explanation see page 92.) 



94 Subsurface Geologic Methods 

Supplementing the preceding data, information on other microfaunas 
and microfloras, lithologies, and mineralogy is also recorded and has been 
found to improve ecologic interpretations. 

OSTRACODES 

Next in importance to Foraminifera in economic micropaleontologic 
analysis are the ostracodes (fig. 40) . These organisms, which are small, 
bottom-dwelling, bivalved crustaceans range in length from 0.5 mm. to 
2.0 mm. They thrive under variable conditions, marine, brackish, and 
fresh water. The most diversified assemblages are those which inhabit 
shallow-marine environments. W. T. Rothwell (Richfield Oil Corp., Cal- 
ifornia), in his studies on the distribution of living ostracodes of Newport 
Bay, California, show them to exist in the following salt-water environ- 
ments: (1) tidal flat, (2) marsh channel, (3) lagoon channel, (4) bay- 
mouth and subtidal channel, and (5) open-sea rocky-tide pool. He further 
comments that the plants appear to be a major biologic factor influencing 
the distribution of the ostracodes at this locale. Further investigations by 
Rothwell of recent forms from samples across the San Pedro Channel 
and from collections along the California coast, indicate that this group 
of animals adjust to various water depths as do the Foraminifera. 

Ostracode carapaces (shells) in some sediments may constitute the 
bulk of the material. Geologically ostracodes extend upward from the 
Ordovician. Those of the Paleozoic assume carapace characteristics that 
are pronouncedly different from those of the Mesozoic and Cenozoic. 

The external or surface pattern of the valves is extremely variable 
and complex. Since most species molt (periodically shed their valves and 
then form new ones), surface features are not constant throughout their 

EXPLANATION OF PLATE 4 

Figure 
1- 4. Hemicy there? californiensis LeRoy var. hispida LeRoy X 29. Fig. 1, right 
valve. Fig. 2, dorsal view. Fig. 3, ventral view. Fig. 4, interior view of 
valve showing hinge structure. 
5- 9. Bairdia verdesensis LeRoy X 24. Fig. 5, right valve. Fig. 6, dorsal view. Fig. 
7, ventral view. Figs. 8, 9, inside view of left and right valves. 

10-13. Caudites fragilis LeRoy X 60. Fig. 10, right valve. Fig. 11, dorsal view. Fig. 
12, ventral view. Fig. 13, inside view of right valve. 

14-18. Hemicythere palosensis LeRoy X 42. Fig. 14, right valve. Fig. 15, dorsal view. 
Fig. 16, ventral view. Figs. 17, 18, interior views of right and left valves. 

19-23. Loxoconcha lenticulata LeRoy X 42. Fig. 20, dorsal view. Fig. 21, ventral 
view. Figs. 22, 23, inside views of right and left valves. 

24-27. Cythereis kewl LeRoy X 37. Fig. 24, right valve. Fig. 25, dorsal view. Fig. 
26, ventral view. Fig. 27, inside view of left valve. 

28-30. Cytheropteron minutum LeRoy X 37. Fig. 28, oblique view of right valve. Fig. 
29, right valve. Fig. 30, posterior view. 

31-39. Characteristic muscle-scar patterns of left valves. Fig. 31, Hemicythere? 
californiensis var. hispida. Fig. 32, Hemicythere? californiensis. Fig. 33, 
Brachycythere lincolnensis. Fig. 34, Hemicythere palosensis. Fig. 35, Bas- 
slerites delreyensis. Fig. 36, Brachycythere driveri. Fig. 37, Bairdia verdesen- 
sis. Fig. 38, Paracypris pacificus. Fig. 39, Hemicythere jollaensis. 



Subsurface Geologic Methods 




Plate 4. Typical marine ostracodes; note the complex hinge structure, 
surface ornamentations, and muscle-scar patterns. 



Subsurface Laboratory Methods 95 

life cycle. Each series of molt valves generally differs somewhat from 
the preceding one. This factor must be carefully considered by micro- 
paleontologists in speciation work, as classification of fossil ostracodes 
depends primarily on shell characteristics. Living forms are commonly 
identified on the basis of the animal appendages. 

The most important elements of fossil ostracodes in descriptive work 
are (1) general shape and outline (side, dorsal, ventral views), (2) rela- 
tive size and overlap of the valves, (3) surface ornamentation, (4) hinge 
characteristics, (5) muscle-scar pattern, and (6) characteristics of the 
interior marginal zones. 

Before a species can be properly described, it is also necessary that 
the male and female individuals be distinguished. This distinction is not 
always possible because of the rarity of valves and slight differences in 
shell structure of the opposite sexes. 

Fossil carapaces may be obtained from the enclosing sediment by 
extraction methods mentioned under "Foraminifera," although the screen 
series generally involves only the 60- and 100-mesh units. 

In determining the vertical and horizontal distribution of ostracodes 
in stratigraphic sequences, the same procedure is followed as in foramini- 
feral studies. Charts are prepared showing species occurrences and abund- 
ences plotted against stratigraphic position. These data are then evaluated 
in terms of zonal intervals. 

Calcareous Algae ^ 

Algae are seaweeds. Some have the ability to secrete lime around 
or within their tissue, and, hence, may be preserved as fossils. These are 
known as the "calcareous algae." During many times in the geologic past 
in many places they have grown so luxuriantly as to build or largely 
build extensive deposits of limestone.^ 

Frequently fragments or even entire specimens occur in well cuttings. 
They can be separated and concentrated in the same manner as most other 
microfossils. Most of them are about the size and shape of fusulinids 
(fig. 41). 

There are a number of groups of calcareous algae, but only three, 
the coralline algae (Corallinaceae) , the siphonous algae (Dasycladaceae), 
and the Charophyta (Chara) are of economic value at present as micro- 
fossils. 

Corallinaceae 

The Corallinaceae (fig. 41, no. 5; fig. 42) are a family of the red 
algae. They develop a very large number of growth forms, the most com- 
mon being (1) forms with thin crusts, (2) forms with crusts from which 
rise mammillary protuberances or small stubby branches and (3) branch- 
ing forms. The branches may be quite extended and may develop as 

° The section "Calcareous Algae" was prepared by J. H. Johnson. 

'Johnson, J. H., Limestones Formed by Plants: Mines Mag., vol. 33, pp. 526-533, 1943. 




Figure 41. Typical fragments from calcareous algae (washed samples). 



Subsurface Laboratory Methods 97 

long, slender needles, may be branching stag-horn types, or may be broad 
and thick, resembling the horn of a moose. 

The coralline algae secrete lime within and between the cell walls, 
thus showing definite microstructure (fig. 42). The various genera are 
separated on the basis of arrangement of cells in the tissue and the char- 
acter and arrangement of the spore cases (conceptacles) . Individual 
species within a genus are separated on the basis of cell and spore-case 
dimensions.^ 

Coralline algae develop in all seas from the poles to the equator 
and from tide level almost to the maximum depth of light penetration. 
Certain genera and individual species have specialized to accommodate 
certain ecologic environs. Coralline algae are recorded from rocks of 
all geologic ages from the Ordovician to the present. They are important 
after Late Cretaceous. Although they grow abundantly in most of the seas 
of the world, they attain their greatest development in the tropics, par- 
ticularly in and around the reefs. 

In the atolls of the Marshall Islands algae are very important as 
builders of reef limestones; locally they form up to 80 percent of the 
reef rock. In the fringing reefs of the Marianas, corals predominate, but 
algae still play a very important part both as contributors to the limestone 
end by acting as binding agents in the reefs. 

Coralline algae produce distinctive fossils, and, because of their 
microstructure, they can be exactly identified in thin sections. Except in 
limited areas, they have not yet been sufficiently studied to determine the 
geologic range of the species. In those areas, however, where they have 
been studied, most of the species appear to have restricted time ranges 
and, hence, have possibilities for serving as guide fossils. They can give 
considerable ecologic information. 

Dasycladaceae 

The Dasycladaceae are small, bushy plants belonging to the green 
algae (fig. 41, nos. 1-4). They consist of a central stem with regularly 
spaced whorls of primary branches radially arranged. These may bear 
secondary and tertiary branches. Calcium carbonate is deposited around 
the central stem and primary branches, forming a shell of variable thick- 
ness. The type of fossil obtained and the amount of structure it exhibits 
depends upon the degree of calcification. The fossils usually appear as 
small rods or club-shaped fragments. A few are spherical; some are 
disc- or umbrella-shaped. They commonly range in length from about 
one-eighth of an inch to more than three-fourths of an inch, although a 
few may grow much larger. Some have characteristic shapes and conse- 
quently are easily recognized, whereas others need to be sectioned for 
identification. Typically the structure consists of a mold of the central 
stem from which whorls of tiny branches develop at regular intervals. 

* Lemoine, Mme. P., Algues calcaires jossiles de I'Algerie:.. Materiaux pour la Carte Geol. de 
I'Algerie, 1^ ser., no. 9, 128 pp., 3 pi., 1939. 




Figure 42. Microstructure of coralline algae in thin sections. No. 1, Lithothamnium 
concretum Howe (X50), Oligocene West Indies (after Howe). No. 2, Mesophyl- 
lum sancti dionysii Lemoine (X50), Miocene, Algeria (after Lemoine). No. 3, 
Archaeolithothamnium floridum (X 75) Johnson and Ferris, Eocene, Florida. 
No. 4, Lithophyllum afif. prelichenoides Lemoine (X75), Miocene, Borneo. 



Subsurface Laboratory Methods 99 

These branches show on the surface of the fossil as rows of small open- 
ings or clusters of openings in regular rows circling around the fossil. 

Ecologically these algae grow in shallow water, on mud flats, or in 
sheltered portions of reefs. Fossil forms occur in many marls, calcareous 
shales, and limestones. 

Geologically they are known in rocks ranging in age from the Early 
Ordovician to the present, although they appear to have made their greatest 
development during the Triassic and Jurassic. 

They have been extensively studied in certain areas of Europe,^ 
where it has been demonstrated that the individual species have short 
geologic ranges; consequently, they can be used as guide fossils. ^° Very 
few have been described from North America, although a careful search 
will undoubtedly show them to be present in many localities. 

Charophyta 

The Charophyta are a distinctive, isolated group of algae (fig. 43) 
usually classed with the green algae (Chlorophyta) . Calcium carbonate 
is usually precipitated around the tips of the branches and spore cases. 
These structures are the only portions of many of the plants that are 
calcified. The spore cases (oogonia) form a distinctive fossil. With their 
spiral ornamentation they are readily recognized, as no other microfossil 
even closely resembles them in structural constitution. 

At the present time the Charophyta inhabit only fresh- and brackish- 
water environs, but during the Paleozoic and possibly during the early 
Mesozoic it appears that some forms may have lived in shallow-water, 
near-shore, marine environments. Their remains occur in great numbers 
in many of our continental deposits,^^ and for some such formations they 
are distinctive microfossils. 

According to Peck and Recker,^^ 

Considering the difficulty of establishing fine divisions in the classification 
of the Charophyta, it is believed that they will never be of value for the 
discrimination of small stratigraphic units. Yet they are of real value in period 
differentiation: continental deposits of Jurassic, Lower Cretaceous, Cretaceous, 
and Lower Cenozoic ages can be readily differentiated on the basis of Charo- 
phyta oogonia. 

Geologically, their remains have been noted in rocks ranging in age 
from the Devonian to the present. 

General 

Fossil algae as a group require a great deal of further study. Recent 
work has shown them to be widespread and abundant. They have useful 

" Morellet, L., and Morellet, J., Les Dasycladacees du Tertiaire Parisien: Soc. geol. France 
Mem. vol. 21, no. 1, p. 43, 3 pis., 1913; Tertiary Siphoneous Algae in the W. K. Parker Collection: 
BritJBh Mus. Nat. History, 55 pp., 7 figs., 6 pis., 1939. 

■"' Pia, J., Die Siphoneae verticillatae vom Karbon bis zuir Kreide : Zool.-bot. Gessel. Wien Abh., vol. 
11, pt. 2, 1920. 

^' Peck, R. E., Fossil Charophyta: Am. Midland Naturalist, vol. 36, no. 2, pp. 275-278, 1946. 

" Peck, R. E., and Recker, C. C, Eocene Charophyta from North America: Jour. Paleontology, 
vol. 22, no. 1, pp. 85-90, 1948. 



100 



Subsurface Geologic Methods 



1^ -i.-:i V ,z' 




Figure 43. Washed samples of typical oogonia of various genera of the Charophyta. 

(After Peck.) 



Subsurface Laboratory Methods 101 

possibilities, both as time-index fossils and as indicators of environment 
of deposition. 

Calcareous algae often coat and fill voids of other fossils. Certain 
algae occur symbiotic with other organisms and produce growths that 
make characteristic and often rather common fossils (e.g., Archimedes). 

Fossil algae are among the oldest fossils known; in fact, they are 
the only group that has been found in considerable numbers in pre- 
Cambrian rocks. 

Diatoms 

Diatoms are microscopic unicellular plants belonging to the phylum 
Thallophyta. The skeleton or frustule, composed of opaline silica, com- 
monly consists of two shallow, disk-shaped halves (example: Craspedo- 
discus) , one of which fits into the other similar to a flat pillbox. Other 
forms are elongate and bilaterally symmetrical (example: Glyphodiscus) 
with respect to an axial strip. Individual specimens may range in diame- 
ter from 0.1 to 0.15 mm. Magnifications up to X 200 are generally re- 
quired to study this group of microflora. It has been estimated that a 
cubic inch of some diatomites (deposits composed essentially of diatoms) 
contains as many as two billion frustules. 

Classification of fossil diatoms is based on the size, shape, and sur- 
face ornamentation of the frustule. The intricacy and complexity of sur- 
face ornamentation of certain species are fantastic and remain surprisingly 
uniform. (See fig. 44). 

Although diatoms live under a wide range of environmental con- 
ditions, they are highly selective as regards sunlight, temperature, salinity, 
turbidity, and percentage of silica in the water. Some species live only 
in fresh water, whereas others thrive only in saline water. Some forms 
attach themselves to objects, although most are of the planktonic variety. 

The fats secreted by diatoms are considered by some petroleum 
geologists to be the source of much of the oil and gas being produced 
from the Miocene sediments of California. 

Diatomite has a world-wide distribution, although the size of the 
individual deposits is not large. Marine Miocene diatomaceous deposits 
occur in California, Maryland, Virginia, Algeria, and Denmark. Large 
fresh-water deposits are found in North America, Europe, and Japan. 
The largest deposit in the world is that at Lompoc, California. This de- 
posit consists of 1,400 or more feet of stratified diatomite in the Monterey 
series of the upper Miocene and covers an area of 4,000 acres. Estimated 
reserves have been placed at 100,000,000 tons. The more important com- 
mercial uses of diatomite are as mineral filters, paint filler, insulation, 
building blocks, and abrasives. 

Diatoms have had a relatively short geologic history ranging from 
the Late Cretaceous to the Recent. They were common in the Cretaceous, 
became prolific in the Tertiary, and reached their highest development 




Figure 44. Typical marine diatoms (single-celled plants possessing a siliceous skeleton) . 
Diatoms occur in all types of aqueous environments. They may be satisfactorily used in 
correlation work, owing to their pelagic adaptability. (After Reinhold.) 



Subsurface Laboratory Methods 103 

during the Miocene. Following Miocene time they decreased rapidly but 
were still present in large numbers. It is impossible to predict how much 
diatomaceous ooze is now being deposited on the present ocean floor. 

Marine diatoms flourish in all latitudes and at all seasons of the year, in 
the warmer and coldest seas. It is well known that they are so abundant in 
frigid zones as sometimes to colour the seas and to tinge with a particular 
hue the blocks of floating ice. 

They are capable of surviving in conditions so diverse, it is difficult to 
believe that any fixed laws of geographical distribution can be discovered 
with respect to them; on the contrary, it might be supposed that the con- 
tinuity of adjacent seas, the surface and submarine currents, the movement 
of tidal waves, the existence of periodical and other winds, the traffic of 
ships and the movement of fishes would all tend to facilitate or bring about 
the mingling of local floras. ^^ 

Diatoms offer great possibilities for establishing local and world- 
wide correlations because of their pelagic character and the siliceous 
nature of their skeletons. Attempts have been made in California and in the 
Netherlands East Indies (Java) to use diatoms for subdividing certain 
parts of the Tertiary sequence. 

Cleaning the diatoms out of different samples is a tedious business. To 
clean a sample of a fossil marl thoroughly, when the particles are solidly 
cemented together perhaps with some volcanic material enclosing the fragile 
microfossils, is not a small task and requires even from the most experienced 
cleaner much skill and patience. Different chemical agents may be used, 
depending on the chemical character of the matrix; too strong solvents may 
destroy the diatoms entirely, which happens sometimes in the most unex- 
pected manner, as any diatomist knows. After the texture of the rock has 
been loosened and something like a dispersion has been obtained, the ma- 
terial may be sifted through very fine sieves, 150- and 300-mesh, and sub- 
jected to several washing operations, decanting after a fixed time. The well- 
known sulphuric-acid treatment with either potassium chlorate or nitrate for 
bleaching is in almost any case inevitable. The whole process cannot be hur- 
ried through and must take its time. It may last several weeks. However cum- 
l ersome the process of cleaning may be, well-cleaned samples are extremely 
important and save much time in the inspection and sorting. Only in the 
case of well-cleaned samples, a complete review of the fossil content is 
possible and the diatoms accessible to further study and to be photographed 
and arranged in neatly mounted slides, which allow a close and thorough 
inspection. Fragments have as a rule to be disregarded except in instances 
where a view of the internal structure is wanted. When incomplete tests are 
inspected, errors will heap up, for studying rare and unknown diatoms from 
badly conserved and imperfectly cleaned fragments leads to false determina- 
tions, which may remain undetected in case of a study purely in search of 
new and strange forms, but when the establishment of fossil lists for strati- 
graphic use is planned, wrong determinations may seriously influence the 
result as to the zonal distribution of the fossils and the geologic age of the 
samples.^'* 



^' Castracane, Fr. D. A., Report on the Scientific Results oj the Voyage of H.M.S. Challenger Dur- 
ing the Years 1773-76: Botany, vol. 2, p. 9, 1886. 

■"^ Reinhold, Th., Fossil Diatoms of the Neogene of lava and Their Zonal Distribution: Geol.-mijnb. 
genootsch. Nederland en Kolonien Verh., Gaol. ser. Deal 12, pp. 43-133, 21 pis., 1937. 




Figure 45. Skeletal remains of Radiolaria. These micro-organisms (single-celled animals) occur 
only in marine habitats and at all latitudes. (After Clark and Campbell, Geol. Soc. America, 
Spec. Paper 39, 1942.) 



Subsurface Laboratory Methods 105 

Radiolaria 

Radiolaria are single-celled, pelagic, open-sea, marine Protozoa hav- 
ing diameters of about 0.1 to 0.5 millimeters. The skeleton or shell 
(fig. 45), consisting of silica which is secreted by the protoplasm, makes 
up an important part of the animal. Little attention has been given to 
fossil radiolarian faunas because of the complex classification, general 
scarcity, and difficulties encountered in procuring well-preserved speci- 
mens. The classification of this group of organisms is based upon the 
structure and composition of the hard parts. 

The majority of the many forms developed within the Radiolaria are 
adaptive to flotation and to lowering the rate of sinking in the water. The 
presence of long spines, many of which are multiplied, of surface spines and 
thorns, and the high development of the hat or cuplike shape of the lattice- 
shell in some are all responses to a reduction in the rate of sinking. These 
structures also allow less effort on the part of the animal to keep it well within 
the photosynthetic zone. . . . Radiolaria occur in all the seas of the world, 
in all climatic zones, and at all depths. However, the Pacific Ocean appears 
to be the richest both quantitatively and qualitatively in these creatures, ex- 
celling both the Indian and Atlantic oceans.^^ 

Radiolaria should be more seriously considered in the future by 
micropaleontologists as a basis for establishing long-range correlations 
because of their pelagic character and the siliceous nature of their skeleton. 

CONODONTS 

Conodonts are small, tooth-shaped, single- or multiple-pointed, or 
platelike fossils occurring locally and in considerable number in Paleo- 
zoic shales (fig. 46). These structures have been interpreted by various 
workers as fish remains, annelid jaws, and gastropod teeth. In greatest 
dimension they range from one to two millimeters. For practical purposes, 
according to Ellison,^*' conodonts can be divided into four groups as 
follows: (1) fibrous, (2) simple cones, (3) blades and bars, and (4) 
platforms. Ellison states further: 

The fibrous and simple cone groups have many genera that serve as ex- 
cellent guides to the Ordovician. The blade and bar group is mainly long 
ranging. However, a few genera serve as guides in the Ordovician, Devonian, 
and Mississippian. The genera in the platform group are best guides to beds 
younger than Silurian. Many of these are remarkably restricted. 

Ellison ^^ concludes that (1) the composition of conodonts is the 
same as the mineral matter in fossil and modern vertebrate hard parts; 
(2) conodonts may be classified as fish or lower vertebrates on the basis 
of their composition, size, shape, assemblage associations, internal struc- 



*' Clark, B. L., and Campbell, A. S., Eocene Radiolarian Faunas from the Mt. Diablo Area, Cali- 
fornia: Geol. Soc. America Special Paper 39, pp. 1-112, 9 pis., 1942. 

^'Ellison, S. P., Jr., Conodonts as Paleozoic Guide Fossils: Jour. Paleontology, vol. 30, no. 1, pp. 
93-110, 1946. 

"Ellison, S. P., Jr., The Composition of Conodonts: Jour. Paleontology, vol. 18, no. 2, pp. 133- 
140, 1944. 



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zoic section. (After Youngquist and CuUison, Jour. Paleontology, 1946.) 




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108 Subsurface Geologic Methods 

ture, associated bone material and jaw parts, and stratigraphic occur- 
rence; and (3) the assignment of conodonts to other zoological groups is 
challenged because these other groups do not possess hard parts of 
similar size and shape that are composed of calcium phosphate. 

A unique method of graphically illustrating the stratigraphic range 
of conodonts is shown in figure 47. 

Otoliths 

Fish otoliths or earstones, which are present in many strata, are 
neglected by many micropaleontologists in stratigraphic and paleontologic 
investigations. These structures, consisting essentially of calcium car- 
bonate and ranging in breadth from 0.1 to 3.0 millimeters, are secreted 
in the auditory system of fish. 

A small one, termed the "lapillus," is formed in a portion of the labyrinth 
known as the "utriculus" ; a second, termed the "asteriscus," is formed in a 
posterior prolongation of the otolith-sac (sacculus), called the "lagena" ; and 
a third, the sagitta, which is the principal earstone which occurs in the sacculus. 
This saggita is the most important and is by far the largest in most cases.^^ 

Plate 5 illustrates the general structural implications of the sagitta. 

According to Campbell, "otoliths are admirably suited to be used 
as a tie between sections located far apart — sections whose comparatively 
local zones are well studied through Foraminifera, ostracoda, and other 
microfossils." 

Otoliths were found to be useful in correlating certain phases of 
the Middle Teritiary sequence of central Sumatra. 

Spores and Pollen 

Fossil plant spores and pollen, which represent a definite part in 
the reproductive cycles of plants, occur in most coals, in many shales, 
and in some of the coarser elastics. British workers have been foremost 



EXPLANATION OF PLATE 5 

Figure 

1-11, 3, 6. Otolithus var. A X 20. Figs. 1, 4, 7, 10, showing variation of inner 
side of the right otolith. Figs 2, 5, 8, 11, showing outer side. Fig. 2, sec- 
tion cut normal to median axis and showing elongate umbilical area. Viewed 
from caudal end. Fig. 6, transverse section showing ovate umbilical area 
and the radiating character of the inner part. (Terminology: DM = dorsal 
margin ; VM = ventral margin ; CE = caudal extremity ; FE = frontal ex- 
tremity ; AR = antirostrum ; R = rostrum ; = ostium ; CA = cauda ; + CA 
= sulcus acusticus; CS = crista superior; A = the area; E — excisura ostii; 
YF = ventral furrow; V = umbilicus.) 

9, 12, 13, Otolithus var. B X 20. Figs. 9, 12, inner side views showing the closed 
character of the sulcus acusticus. Observe the prominent projections along 
the ventral margin, which constitutes a distinctive feature of the variety. 
Fig. 13 illustrates the radial character as well as the concentric growth lines. 

'' Campbell, R. B., Fish Otoliths, Their Occurrence and Value as Stratigraphic Markers: Jour. 
Paleontology, vol. 3, no. 3, pp. 254-279, Sept. 1929. 



Subsurface Geologic Methods 



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Plate 5. Showing the structural constitution of fish otoliths (ear bones), 
Locally these structures have been used in correlation work. 



Subsurface Laboratory Methods 



109 




J'lGURi 48. Carboniferous plant spores from Daggett County, Utah. Numbers 1-4, 
Triletes; 5-10, Punctati; 11-15 Granulati; 16-17, Denso-sporites. All about 400X, 
except 1-3 (SOX). (From Schemel, Jour. Paleontology.) 



110 Subsurface Geologic Methods 

in using spore and pollen data for correlating Carboniferous strata, but 
little work has been done in the United States on these microfossils. An 
attempt is now being made by the Creole Petroleum Corporation in Vene- 
zuela to use pollen analysis in correlation problems. 
According to Wilson/^ 

Spores and pollen meet the requirements for correlation work and have 
proved their worth, at least in a limited capacity. The chemical nature of 
spores and pollen is such that most species are resistant to decay. They are 
varied in their structure and ornamentation, which allows the easy identification 
of many groups of plants and frequently permits specific determination. They 
are small and disseminated mainly by air currents. Many species are widely 
and uniformly spread throughout a region and the sediments in which they 
may be preserved are numerous. . . . Correlation of strata by fossil spores 
and pollen has at least three factors of biological background that aid in 
the accomplishment of results. These are (1) the evolution of floras, (2) the 
geographic distribution and migration of floras, and (3) the edaphic ecological 
relations of plants. 

Methods pertaining to collection, maceration of samples, and micro- 
scopic techniques followed in spore and pollen analysis are clearly treated 
by Wilson.-" As regards correlation procedures and problems, Wilson 
presents the following discussion: 

Spores and pollen that occur in the samples may be designated as either 
dominant or accessory species depending upon their abundance. Usually the 
dominant species occur in the frequency of five or more percent of the total 
count in the sample. The accessory species are less frequent and sometimes 
are not represented by more than a single specimen in a count of one thousand. 
With the present state of knowledge concerning the distribution of the fossils 
the dominant species are of most value in correlation studies, but as the vertical 
and geographical ranges are better understood the accessory species will be- 
come more significant. This assumption is based upon the possibility that 
some of the spores or pollen may have come from plants of restricted vertical 
range in the rocks, of specific paleoecological conditions, or of important index 
species that produced comparatively few spores or pollen. 

For stratigraphic work the fossils ocurring in the samples may be divided 
into the following three groups: (1) knowns, (2) unknowns, and (3) those 
broken beyond recognition. The knowns are those fossils that are recognized 
as species already described or tentatively given descriptions. They are re- 
corded and used in correlation work. The unknowns are those fossils that are 
undescribed and are usually not abundant. Some of these later may be found 
to be important dominant or accessory species, but in the preliminary work 
usually are not used. They are lumped under the heading of "unknowns." 
In the counts it is desirable to have a record of the number of unknown forms 
and to prepare descriptive illustrations of the more diagnostic types. Those 
fossils that are broken to the point where they cannot be identified with de- 
scribed species are ignored in the counts. It is assumed that all species except 
certain resistant forms will break down in approximately the same ratio or in 
specific ratios that will not materially affect the final count. 

The number of spores and pollen that should be counted for satisfactory 
correlation work appears to be based on the use to which the study is directed, 

^'Wilson, L. R., The Correlation of Sedimentary Rocks by Fossil Spores and Pollen: Jour. Sedi- 
mentary Petrology, vol. 16, no. 3, pp. 110-130, 1946. 
2» Wilson, L. R., ibid. 



Subsurface Laboratory Methods 111 

and the number of species present in samples. In peat investigations, pollen 
of tree species are usually the only ones used and the conclusions are usually 
based upon a count of 200 fossils. In peat seldom more than a dozen plant 
species are present and frequently the total number of tree pollen species is 
not more than six in any one level. In coal and shales studies the paleontologist 
uses all types of spores and pollen and seldom restricts his studies to tree 
pollen. For this reason the number of species is often several times the number 
encountered in peat. Consequently, it is desirable to examine and count a 
greater number of individuals. What this count should be is not established 
and differs widely among workers. . . . 

Graphic treatment of the counts is probably the best method of demon- 
strating plant microfossil correlations. Line graphs were early used in peat 
pollen studies to show paleoecological succession, but these have been largely 
replaced by bar graphs of several types. . . . 

Two types of graphs can be constructed for correlation purposes. These 
are channel-sample graphs and horizon-segment graphs. The former is the 
better type for direct correlation purposes, if the strata involved are not 
more than several feet thick. In thick coal seams, channel sample correla- 
tion becomes difficult. Where possible, a coal seam should be divided into 
segments separated by shale or pyrite partings. These partings are often of 
extensive areal extent and make good natural boundaries. In shale de- 
posits lithological types should be used to limit the extent of the sample. 
In order to determine paleoecological succession within a rock member, 
contiguous thin vertical samples must be studied. The graphed results will 
show the percentage of the fossils at successive levels. Close correlation of 
comparable measured horizons has not yet shown exact percentages of identical 
species, but successional trends are usually indicated, if the section is viewed 
as a whole, and as such have paleoecological and stratigraphic value. 

How similar the percentages of each fossil species must be in strata to 
indicate correlation is a question of considerable pertinence. Experiments 
with seams of coal have shown that the dominant fossils will frequently vary 
between five and ten percent in a 200-fossil count, if the samples are collected 
several miles apart, or the maceration process has not been uniform. In the 
first instance, areal distribution of the ancient vegetation apparently is a 
factor, or the coal seam was thicker or more completely represented at one 
locality than at another. In the second instance, where uniformity of macera- 
tion is not attained, fossils with various thicknesses of spore or pollen coat, 
or great differences in size, will appear in variable percentages. An effective 
method of combating such problems is to divide the sample in the course of 
its preparation and allow additional time for each portion of the sample. 
Uniformity can be checked with test slides by using the corrosion of certain 
species of fossils as an index for uniformity of maceration. 

In conclusion, it might be stated that results have been attained with 
fossil spores and pollen which show conclusively that all coals and shales thus 
far studied can be assigned within the limits of geological periods, and that 
various strata can be separated from each other by their spore and pollen 
facies, or specific abundance of various species. It would seem that plant micro- 
fossils have great future scientific and economic value as the science develops 
and broadens. 

Grass Seeds 

Several years ago Elias "^ published a noteworthy paper pertaining 
to prairie-grass seeds of the Late Tertiary deposits of the Prairie States. 

Elias, M. K., Tertiary Prairie Grasses and Other Herbs from the High Plains : Geol. Soc. America 
Special Paper 41, pp. 1-176, 1942. 



112 



Subsurface Geologic Methods 



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Figure 49. Fossil and recent grass seeds. (From Elias, Geol. Soc. America Special Paper 41, 
1942.) (For explanation see page 113.) 



Subsurface Laboratory Methods 113 

(See fig. 49.) In abstract he says: 

The most common among these fossil seeds show close relation to the 
most typical modern prairie grasses. . . . Comparative study of the fossil and 
living forms reveals evolutionary trends of the seeds of prairie grasses. The 
rather small and generalized Miocene ancestor gave rise to greatly diversified 
Pliocene and Recent species. The seeds of these include small and very large, 
very slender, and very stout forms, all of them variously adapted for protection 
against drought and for more efficient dispersal. Abundance, good preserva- 
tion, and rapid ecologic and evolutionary changes make grass seeds the best 
index fossils for subdivision of the continental Late Tertiary rocks. 

Fish Scales 

Fish remains, especially fish scales, commonly occur in sediments. 
In the Pacific Coast Tertiary section, according to David,-^ a 

. . . great number of fish scales can be used characteristically as index or 
marker fossils. Others occur in well-marked abundance zones, indicating in 
that way horizons of definite age and making it possible to correlate quite 
accurately. . . . Scales of different kinds of fishes show innumerable variations 
of their ornamentation, of the designs and angles formed by the fingerprintlike 
impressions that mark the scales. . . . Herrings and round herrings are the 
most abundant scales through the Tertiary. ... A number of very distinct 
scales are of importance for paleoecological determinations. . . . These fossil 

EXPLANATION OF FIGURE 49 

Magnification 
Nos. 1, 3, and 10, X 3; No. 4, X 33; No. 5. X 29; Nos. 6-9, 11-16, X 15. 
Fossil Panicum elegans early mutation nebraskense ; Panicum elegans; Setaria 
chasea. Living Panicum angustifolium; Setaria chasea. Living Panicum angus- 
tifolium; Setaria crusgalli; S. glauca. 
Nos. 

1,2. Panicum elegans Elias, early mutation nebraskense Elias n. mut.; basal part 
of Biorbia fossilia zone, Ash Hollow formation, Ogallala group; east of rail- 
road bridge, Wi miles west of Wauneta, Chase County, Nebraska; middle 
Pliocene. 

3, 4. Panicum elegans Elias. 3, Autotypes, about seventy feet below the top of the 
local section of Ogallala, two miles east-northeast of Ogallala, SE%, sec. 33, 
T. 14 N., R. 38 W., Keith County, Nebraska. Both from upper part of Biorbia 
fossilia zone. Ash Hollow formation, Ogallala group; middle Pliocene. 4, 
Holotype, showing epidermis of palea, from about forty feet below algal 
(Chlorellopsis) limestone (at top of Ogallala group), sec. 21, T. 14 S., R. 39 
W., Wallace County, Kansas. 

5. Living Panicum angustifolia Elliott; eastern United States. 

6-8. Living Echinochloa crusgalli (Linne) Beauvois; from Mexico. 
10-14. Chaetochloa chasea Elias, n. sp.; syntypes; west of United States Department 
of Agriculture Experiment Station at North Platte, Lincoln County, Nebraska. 
Upper portion of Ogallala group above Biorbia fossilia zone. Collected by 
H. E. Weakly. 11, showing tripartite apex of the lemma; back view. 12, show- 
ing frontal view of upper part of hull, palea parting from lemma; uppermost 
middle Pliocene or upper Pliocene. 15-16, living Chaetochloa glauca; from 
near Almera, Himalaya Mountains. Note slight parting of lemma and palea 
in No. 15; also similar type of sculpture of both lemma and palea with 
Setaria cahsea Flias. 



^^ David, L. R., How Fossil Fish Remains Have Been Used in Pacific Coast Stratigraphy: Petroleum 
Eng., vol. 18, no. 8, pp. 104-113, May 1947. 



114 Subsurface Geologic Methods 

fishes are highly significant in the study of stratigraphy, sedimentation, and 
paleontology. 

From these comments it is obvious that paleoichthyology has a definite 
place in stratigraphic and correlation work, and in the future should be 
considered more seriously by micropaleontologists. 

Suggested Micropaleontologic Studies 

For more adequate interpretation of micropaleontologic data, a 
thorough understanding of present-day biotopes and ecologic relationships 
is required. Integrated detailed investigations of modern faunas and floras 
would furnish much information that could be of great value to paleon- 
tologists. Contributions by Natland,-'^ Norton,-^ Phleger,-'^ and Cushman ^^ 
in this field are basic. Similar studies should be instigated and liberally 
supported by the oil companies and educational institutions. 

A systematic review of the biologic literature would undoubtedly re- 
veal information that would be of considerable value to the paleontologist.. 

Detailed examination and recording of microfaunas (Foraminifera, 
ostracodes, diatoms, Radiolaria, spores, pollen, and the like) in controlled 
stratigraphic sections should be continued with maximum effort. More 
study should be devoted to phylogeny, taxonomy, and habitats of recent 
microfaunas and microfloras. 

Pelagic microfaunas and microfloras are scheduled to play an im- 
portant role in the future in establishing long-range geologic correlations. 
To evaluate fossil suites more adequately, studies of related recent types 
should be started. Geographic distribution patterns of these "floaters" in 
modern seas should be prepared and analyzed in relation to water and 
wind currents, salinity, turbidity, food supply, temperature, and land 
barriers. 

More data are required on the rate of accumulation of remains in 
micro-organisms in recent sediments. Detailed investigations of microfos- 
sils of the various periods and epochs are critically needed and should 
be periodically evaluated and published. Total faunas should be illus- 
trated and each species carefully described. Too frequently have incom- 
plete faunas been described in the literature during the past. Published 
faunas should be considered stratigraphically as well as paleontologically. 

Commercial Micropaleontologic Laboratories 

The routine of micropaleontologic laboratories depends on the prob- 
lem involved (surface or subsurface, detailed or reconnaissance, research 



^' Natland, M. L., The Temperature and Depth Distribution of Some Recent and Fossil Foraminifera 
in the Southern California Region: Scripps Inst. Oceanography Tech. Ser. Bull., vol. 3, no. 10. pp. 225- 
230, I tab'.e, 1933. 

"^ Norton, R. D., Ecologic Relations of Some Foraminifera: Scripps Inst. Oceanography Tech. Ser. 
Bull., vol. 2, pp. 331-388, 1930. 

^ Phleger, F. B., Jr., Foraminifera of Submarine Cores from the Continental Slope: Geol. Soc. 
America Bull., vol. 50, pp. 1395-1422, 1939. 

-° Cushman, J. A.. A Study of the Foraminifera Contained in Cores from the Bartlett Deep: Am. Jour. 
Sci., vol. 239, pp. 128-142, 1941. 



Subsurface Laboratory Methods 



115 



or economic), the personnel, the expenditure permitted, and the work out- 
put. Methods applied in a laboratory of one area may vary considerably 
from those followed in a laboratory of another. Laboratory procedures of 
various companies may differ widely within the same area. Some com- 
panies support only a one-man laboratory; others support laboratories 
having personnel up to 120 men working in three eight-hour shifts. Op- 
erations for a medium-sized laboratory are shown in figure 50. 

Routine operations of oil-company laboratories are much the same in many 
parts of the world in spite of considerable diversity in the nature of the 



PRELIMINARY LABORATORY EXAMI 



NATION [- 



FIELODESCRIPTION 



LOCALITY MAP 



PORTION FOR 
EXAMINATION 



I PORTION FOR 
STORAGE 



SUPPLEMENTAL NOTES 



Ul 



I SIEVES 1-'^ 

'l ' ■ ' ' 

'MEDIUM I I FINE 1 ICOARf^Fl- 



OFFICE SUMMARY 



MINUTE ORGANISMS 



I discardI 



LARGE ORGANISMS 

(Sm^// AToUtJScs, 

Ori/Axc/s, etc ) 



JFi lterHoven kIconcentratorI I 



T \ _JL_, L , 

I ICONCENTRATESl | TAILINGS^ 



|_ "-H MICRO-EXAMINATIONS I 



PETROGRAPHIC 
EXAMINATION 



RECORD 

OF 

DATA 



Figure 50. Flow chart of operations in a medium-sized micropaleontologic laboratory. 

(After Driver.) 



problems confronting the paleontologists and their differences in viewpoint. 
Whatever the area and whatever the details of procedure, the operations of 
these laboratories may be divided into two parts: the preparation of material 
for examination, and the work of the microscopist in studying and reporting 
on the material. Systematic orderliness, fine equipment, and special techniques 
help speedily to achieve the primary objective — the accurate correlation of 
strata. 

The procedure in a commercial laboratory is not automatic, stereotyped, 
unthinking mass production of data. It is, on the contrary, research by trained 



116 Subsurface Geologic Methods 

persons who are thoroughly familiar not only with their objectives but also 
with the difficulties that confront them. ... It is true that oil company micro- 
paleontologists have invented some useful equipment but most of it is of minor 
importance. Principally they have adopted inventions made by others; they 
have modified equipment already manufactured; they have organized material 
and personnel for systematic, rapid study. In other ways, however, their 
contributions have been of great importance; by their refined correlations of 
strata, they have aided in the discovery of oil and in the unraveling of the 
complicated history of the earth.^^ 



DETRITAL MINERALOGY 
GORDON RITTENHOUSE 

This section outlines briefly the methods used to study the mineralogy 
of subsurface samples, some of the limitations of these methods, and the 
uses to which the results may be put. Much has been written on the meth- 
ods of studying and interpreting sedimentary rocks. Agreement is far 
from unanimous on either the best methods of study or the meaning of 
the information after it has been obtained. In this section the writer has 
attempted to emphasize those methods that he thinks have been or will be 
most useful in practical subsurface work and to present those interpreta- 
tions of data that seem to him most logical and reasonable. 

The study of the mineralogy of subsurface samples is helpful in one 
or more of the following: (1) correlating or identifying key beds or 
producing horizons, (2) determining the extent, thickness, and lithologic 
variations of beds or groups of beds, (3) selecting the best methods of 
completing wells, (4) selecting the best methods for secondary recovery, 
(5) determining the source or sources of various beds or various parts 
of the same bed, and (6) determining the geologic history as it can be 
deduced from the composition and physical properties of the rocks and 
from the relations of different beds to one another. 

Much of the work on the mineralogy of subsurface samples has been 
primarily descriptive: the composition, texture, color, and other obvious 
characteristics are described in varying degrees of detail, and the rock is 
given a name. Much exceedingly valuable information has been obtained 
and will continue to be obtained in this way. One has only to consider 
the great progress that has been made in subsurface geology during the 
past three .decades to appreciate the value of such descriptive work. Al- 
though geophysical logging methods have recently supplanted sample ex- 
amination in part, subsurface samples will continue to be used extensively 
in the future. 

As stratigraphic traps or combined stratigraphic and structural traps 
become increasingly important sources of oil and gas, interpretation of 
subsurface samples in terms of sources of sedimentary materials, the 

^^ Schenck, H. G., and Adams, B. C, Operations of Commercial Micropaleontological Laboratories: 
Jour. Paleontology, vol. 17, no. 6, pp. 554-583, 1943. 



Subsurface Laboratory Methods 117 

conditions of transportation and deposition, and post-depositional changes 
may be expected to increase in importance. Sedimentary rocks have many 
properties from which their past history may be deduced with varying 
degrees of success. Most rocks are made up of a large number of grains 
and these grains have the individual properties of size, shape, roundness, 
surface texture, orientation in space, and a variety of other properties 
that are dependent on and can be grouped under composition. In the 
aggregate the particles have mass properties. These include average size 
and a spread about that average, average shape, roundness, and orienta- 
tion, and a spread about these averages. Porosity, permeability, color, 
mud cracks, bedding, ripple marks, and the kind and degree of induration 
are some of the many other mass properties. 

Of these properties, some, like composition, may be inherited from 
the original source material; some, like shape and roundness, were devel- 
oped as the particles were transported from the source to their place of 
deposition; some, such as orientation, size, and bedding, reflect the envi- 
ronment of deposition; and some, like porosity and permeability, at least 
in part reflect post-depositional changes. It is with the study and inter- 
pretation of these individual and mass properties that this section is pri- 
marily concerned. 

Choice of Methods 

Many methods are used to study the mineralogy of subsurface sam- 
ples. Some properties of the samples can be determined by megascopic 
examination, supplemented by simple chemical and physical tests. Other 
properties can be studied best with a binocular or petrographic micro- 
scope. Certain minor constituents may be concentrated as insoluble or 
heavy-mineral separates before binocular or petrographic examination. 

In selecting methods of study careful consideration must be given 
to the objectives of the investigation and to the type of data required to 
achieve these objectives. Methods that provide satisfactory information 
for one problem or one area may be entirely unsuited to another problem 
or another area. The hazards of unthinking application of techniques and 
methods cannot be overemphasized. 

Naturally the type, number, size, and location of samples that are 
available are controlling factors in outlining a program of investigation 
and in choosing methods of analysis. Rotary cuttings, cable-tool cuttings, 
and cores may yield diff"erent types of data. Given certain samples, how- 
ever, choice of methods depends on the answers to two questions, namely, 
what methods will provide the data needed to solve the problem under 
consideration, and which method will yield the information with the 
minimum expenditure of time and money. 

As an example, many correlation problems may be solved by rapid 
megascopic or binocular examination of a few samples or a series of 
samples. Where this is possible, further extensive laboratory examination 



118 Subsurface Geologic Methods 

lo obtain information about grain size or heavy-mineral content is time 
wasted insofar as correlation is concerned, although such examinations may 
yield valuable information on other problems. In general, one should 
start with the simplest and fastest procedure and try successively more 
complex and slower procedures until one is found that will provide the 
needed information. A little forethought and a preliminary examination of 
a few representative samples may not only save time and money but 
determine the ultimate success or failure of an investigation. 

Characteristics of Different Types of Subsurface Samples 

Subsurface samples are usually of three types, namely, rotary cuttings, 
cable-tool cuttings, and cores. The way in which these three types of 
samples are procured limits the amount of data than can be obtained from 
them and the interpretation of that data. The brief and perhaps over- 
simplified explanation that follows outlines some of the major points to 
be considered in studying these three types of samples. 

In rotary drilling the cutting action is provided by the abrasion and 
downward pressure of a steel bit attached to a hollow drill pipe. Drilling 
mud is pumped down through the drill pipe and returns to the surface in 
the space between the drill pipe and the sides of the hole. The drill 
cuttings carried to the surface in the drilling mud are usually separated 
from the mud as it passes over a vibrating wire screen or through a shaker. 
Samples of the accumulated cuttings are taken at intervals. Rotary cut- 
tings have the following characteristics that limit their usefulness: 

1. Some rocks, particularly poorly cemented sands or bentonitic 
shales that break into small fragments or disintegrate into mud, pass 
through the screen or shaker and are not present or are present in only 
small amounts in the samples. Sometimes more representative cuttings 
can be obtained by diverting a part of the mud and cuttings into a con- 
tainer and washing out the drilling mud after the cuttings have settled. 

2. Because rotary wells usually are not cased until drilling is com- 
pleted, cavings from above may form a very large porportion of the 
sample, especially when mud consistency has not been carefully controlled. 

3. Large and small cuttings, or cuttings of different specific gravity, 
tend to be carried upward at different rates, and as a result the cuttings 
from different horizons are mixed. 

4. Owing to the time required for the cuttings to be carried from the 
bottom of the hole to the surface, the samples come from a somewhat 
higher horizon than is being drilled at the time they are collected. A 
depth correction may or may not be made at the well. A rule-of-thumb 
correction of ten feet per 1,000 feet of depth is commonly used. 

5. The drill cuttings are usually less than three-eighths inch in dia- 
meter. Hills,^^ whose recent paper on examination of subsurface samples 

^' Hills, J. M., Sampling and Examination of Well Cuttings: Am. Assoc. Petroleum Geologists Bull., 
vol. 33, no. 1, pp. 73-92, Jan. 1949. (See pp. 344-364 of this Symposium). 



Subsurface Laboratory Methods 119 

gives more information than is possible here, notes that much larger 
cuttings can be obtained by reverse circulation. 

In cable-tool drilling, a chisel-shaped bit attached to a cable is al- 
ternately raised and dropped. After drilling five feet or so, the bit is 
removed from the hole and the accumulated cuttings are bailed out of the 
hole with an elongated bucket having a valve at the lower end. The bailer 
is emptied into a bucket or barrel and the cuttings are washed free of mud. 
The hole is cased past places where excessive caving occurs or where ex- 
cessive water enters the hole. Cable-tool cuttings have the following char- 
acteristics that limit their usefulness: 

1. Some contamination of the cuttings from the uncased parts of 
the hole occurs. Usually some material from five to ten feet above is in- 
cluded because the hole is widened as drilling proceeds. In general, cable- 
tool cuttings are less contaminated and more representative of the interval 
drilled than are rotary cuttings. 

2. Stretching of the drilling cable results in inaccuracies in depth 
measurements up to twenty feet or more. Steel-line measurements at in- 
tervals provide a method of correction. 

3. The cuttings are usually less than three-eighths inch in diameter. 

4. Poorly cemented pay sands may be blown out of the hole by 
escaping gas, and consequently samples, if any, from such horizons may 
not be representative. 

5. Electric logs cannot be made on cased holes and therefore are 
not available to supplement the sample studies. 

6. In some areas cable-tool drillers are not so careful in collecting 
and washing samples as are rotary drillers. 

Cores may be collected with rotary, cable-tool, or core-drilling ap- 
paratus. The chief disadvantages of coring are (1) high cost, (2) selective 
loss of weaker or more soluble rocks, and (3) storage and transportation 
of the cores. Improvements in coring methods are reducing costs and in- 
creasing core recovery. 

Megascopic and Binocular Examination 

To a certain extent, the old saying "the closer you look, the less you 
see" is applicable to some properties of sedimentary rocks. The color, 
texture, and composition of cuttings and cores and the bedding, cross- 
bedding, and porosity in cores can often be determined better and faster 
by megascopic examination and simple chemical and physical tests than 
in other ways. 

By placing ten to twenty cable-tool or rotary samples on sheets of 
paper or in cardboard or metal trays and observing their appearance mega- 
scopically, major changes in color, texture, and composition are readily 
apparent if the samples have been washed clean of drilling mud. The 
boundaries of beds or formations that differ markedly from those above 
or below often can be determined quite accurately. After the major breaks 



120 



Subsurface Geologic Methods 



have been picked, attention should be directed to the character of the 
sediments in each unit and to the nature of the transition between units. 
Some of these characteristics can be determined by megascopic examina- 
tion; others can be determined better with the binocular microscope. The 
following discussion applies to both megascopic and binocular examina- 
tion of samples. 



CLASTIC 



CRYSTALLINE 



PEBBLES 



GRANULES 



VERY COARSE 



COARSE 



MEDIUM 



FINE 



VERY FINE 



SILT 



CLAY 



10. 



.01 



.001 



COARSE 



3.2 



FINE 



COARSE 



56 



MEDIUM 



.32 



FINE 



18- 



VERY FINE 



COARSE 



096 



MEDIUM 



032 



FINE 



.018- 



VERY FINE 



COARSE 



.0096 - 



0032 



MEDIUM 



FINE 



.0018- 



VERY FINE 



VERY FINE 



CRYPT06RAINED 



MEGACRYSTALLINE 



.02 



COARSELY 
CRYSTALLINE 



MEDIO - 
CRYSTALLINE 



FINELY 
CRYSTALLINE 



CRYPTO- 
CRYSTALLINE 



MAT 
( INVISIBLE ) 



Figure 51. Suggested grade scales for clastic and crystalline sedimentary rocks. 



Subsurface Laboratory Methods 121 

Color 

In the past color description of sedimentary rocks has been a more 
or less "every man for himself" proposition. Not only have different men 
described the same rock differently, but the same man often cannot dupli- 
cate his own descriptions. Recently, DeFord ^^ discussed previous work 
and made suggestions that have been partly responsible for the prepara- 
tion and publication of a "Rock Color Chart." ^'^ Although the writer has 
not yet had the opportunity of using this chart, it or one of the other 
color standards recommended by DeFord should be used in describing 
color. 

Grain Size 

Most sediments are composed of clastic particles that have been 
transported to their place of deposition, of crystals that have grown in 
situ, or a combination of clastic particles and crystals. Rocks containing 
fossils that have not been transported might be considered as another 
group. No size terms for either clastic or crystalline sedimentary rocks 
have been generally accepted for subsurface work. Thus, it is important, 
whatever method of size classification is adopted, to indicate what that 
scale of measurement is. 

For clastic rocks, exclusive of clastic carbonates, the terms "conglom- 
erate," "sand" (sandstone), "silt" (siltstone), "clay" (claystone), and 
"shale" are widely used, but different workers define and use them differ- 
ently. The Wentworth grade scale, shown in figure 51, has probably been 
used most widely, and has the advantage of separating sand and silt at the 
one-sixteenth-millimeter size. Above this size the individual grains are 
clearly visible to the naked eye; below this size they are indistinct or in- 
visible. This size also is approximately the point of separation between 
wash-load and bed material in some present-day streams and may have 
rather widespread genetic significance in ancient deposits. One-sixteenth 
millimeter also is approximately the size that separates the productive 
from nonproductive sands in some oil and gas fields. 

For most subsurface work, comparison with a standard set of samples 
or sieve separates mounted in glass vials or on microscope slides (fig. 52) 
will permit adequate description of size. The description should indicate 
the average size of grain and the spread or "sorting" about that average. 
In combinations of sand with pebbles, silt, or clay, the main constituent 
should determine the rock name, and the minor constituent should be 
used as a modifier, i.e., silty fine sand, fine sandy silt, etc. 

The size of crystalline sediments is commonly recorded as "coarse," 
"medium," "fine," "very fine," "cryptocrystalline," or "lithographic," but 

" DeFord, R. K., Rock Colors (review) : Ami. Assoc. Petroleum Geologists Bull., vol. 28, no. 1 
pp. 128-137, Jan. 1944. 

^ Goddard, E. N., Chairman Rock-Color Chart Coram., Rock Color Chart: U. S. Geol. Survey Spec. 
Pub., 1948. (For sale by Nat. Research Council, Div. of Geology and Geography, 2101 Constitution Ave., 
Washington 25, D. C, $5.50.) 



122 Subsurface Geologic Methods 

no general agreement exists as to what sizes these terms include. Recently 
the tentative size scales shown in figure 51 have been proposed by De 
Ford ^^ and Hills'^" (West Texas Geological Society) for carbonate rocks. 
These are recommended for consideration and trial. 

For rocks that are combinations of clastic grains and crystals, the 
dominant constituents should determine the main name, and the minor 
constituent should determine the modifier. To each, the size terms in figure 
51 may be applied. A fine sandy, medium-paurograined dolomite would 
be a dolomite with crystals between .056 and .032 mm. in diameter that 
contained less than 50 percent of fine sand. In those combinations in 
which this terminology may be cumbersome, especially when the composi- 
tion of the sand is indicated, the modifier may be added as a qualifying 
phrase. In those cases where more accurate designation of the proportion 
of the constituents is desirable, the estimated percentages of them may be 
indicated. 

As Hills has noted, experience has shown that full descriptions, of 
subsurface samples have saved much time and money, whereas meager 
descriptions have necessitated one or more re-examinations of the cuttings. 
Hills also recommends that wildcat wells and pay sections be described 
in more detail than field wells or long sections of comparatively insig- 
nificant beds between key beds. 

Shape and Roundness 

Shape (sphericity) and roundness of sedimentary grains are inde- 
pendent properties and should not be confused. Sphericity is a measure 
of the approach to spherical form and may be expressed roughly as a 
ratio between the diameters of the grains. In contrast, roundness is a 
measure of the angularity of the corners and edges of the grains. 

In most subsurface samples the particles or crystals are monomineral, 
and their shape is largely dependent on their composition. Therefore, 
separate-shape terms usually are not necessary when the mineral compo- 
sition and texture are indicated. For rock fragments, the shape terms sug- 
gested by Krynine,^^ as slightly modified by the writer, are indicated. 
The methods used for more detailed measurements of shape and roundness 
than ordinarily needed are outlined elsewhere in this section. 

Equant — Length of grain is less than 1^ times its width and thickness. 

Prismatic — Length of grain is 1^ to 3 times its width and thick- 
ness. 

Tabular — Length and width of grain are 1^ to 3 times its thickness. 

Acicular — Length of grain is more than 3 times its width and thick- 
ness. 

Platy — Length and width of grain are more than 3 times its thick- 
ness. 



2" DeFord, R. K., op. cit. 
'- Hills, J. M., op. cit. 

^^ Krynine, P. D., The Megascopic Study and Field Classification of Sedimentary Rocks: Jour. Geol. 
ogy, vol. 56, no. 1, pp. 130- 165, Jan. 1948. 



Subsurface Laboratory Methods 



123 



Three or more of the terms, "well-rounded," "rounded," "sub- 
rounded," "subangular," and "angular," are generally used to describe 
roundness or angularity in megascopic and binocular examinations. There 
has been no general agreement as to the meanings of these terms. For 




Coarse Silt 




Very Fine Sand 




Fine Sand 




Medium Sand 



Figure 52. Textural standard for sam- 
ple work. Dots do not represent true 
grade size. 



124 Subsurface Geologic Methods 

most megascopic and binocular examinations of subsurface samples, the 
following definitions as proposed by Russel and Taylor ^^ are recom- 
mended: 

Angular — Showing very little or no evidence of wear. Edges and corners 
are sharp. 

Subangular — Showing definite effects of wear. The grains still have their 
original form, and the faces are practically untouched, but the edges and cor- 
ners have been rounded off to some extent though the angles between the faces 
may still be sharp. 

Subrounded — Showing considerable wear. The edges and corners are 
rounded off to smooth curves, and the area of the original faces is considerably 
reduced, but the original shape of the grain is still distinct. 

Rounded — Original faces almost completely destroyed, but some compara- 
tively flat faces may be present. There may be broad re-entrant angles between 
remnant faces. All original edges and corners have been smoothed off to rather 
broad curves. 

Well-rounded — No original faces, edges, or corners remain. The entire 
surface consists of broad curves; flat areas are absent. However, the original 
shape of the grain may be suggested by its present form. 

It is particularly important to note whether the sedimentary particles 
have about the same rounding, or whether there is a mixture of angular 
and well-rounded grains. "Mixed" rounding of the grains of a sand 
generally indicates that the grains have not had the same past history 
and may have been derived from different sources. Because rounding of 
quartz sand grains is a very slow process, particularly for grains of fine 
or very fine size, rounded or well-rounded grains probably indicate deriva- 
tion from a pre-existing sediment. 

The deposition of quartz cement in a quartzose sandstone usually 
modifies the original shape and roundness of the grains. Shape and 
roundness measurements must be made on the original grains to be signifi- 
cant. Any sandstone that sparkles in the sunlight or is seen to reflect 
light from flat faces when viewed under the binocular microscope usually 
has had the original shape and roundness of the grains modified by cement. 

Surface Texture 

The surface textures of sand grains may be seen megascopically or 
inferred from the general appearance of the rock but usually can be 
seen more clearly through a binocular microscope. A classification of 
surface textures prepared by Williams ^^ and recommended here is as 
follows: 

Luster (grains also may be smooth or rough). 

A. Dull 

B. Polished 

Relief (grains may be dull or polished). 
A. Smooth 



'* Russell, R. D., and Taylor, R. E., Roundness and Shape of Mississippi River Sands: Jour. Geology 
vol. 45, no. 2, pp. 225-267, Apr. -May 1937. 

^ Williams, Lou, Classification and Selected Bibliography of the Surface Textures of Sedimentary 
Fragments: Nat. Research Council Comm. on Sedimentation Rept. 1936-1937, pp. 114-128, 1937. 



Subsurface Laboratory Methods 125 

B. Rough 

1. Striated 

2. Faceted 

3. Frosted 

4. Etched 

5. Pitted 

One must be careful in interpreting the meaning of surface textures. 
If transportation has not been sufficiently long or rigorous, a pre-existing 
surface texture may be modified only slightly and consequently may be 
an inheritance from a pre-existing rock. Also the original surface texture 
of detrital grains may be modified either by solution or by the addition 
of cement of the same composition after deposition. The presence in a 
sand of a wide range of surface textures probably means a mixed source 
for the sand. 

Orientation 

Although it is an important property, the arrangement of sedimen- 
tary particles in space does not lend itself to megascopic or binocular 
examination or description in most subsurface work. The small size of 
clay and silt particles, the small size and lack of known orientation of 
cuttings, and the tendency of many sands to break down to individual 
grains make orientation study of rotary and cable-tool cuttings difficult 
or impossible. In oriented cores that contain pebbles or coarse sand the 
orientation of the particles may give clues concerning the direction of 
flow of the depositing currents. The imbricate or shingled arrangement of 
flattened pebbles shows the direction of current movement. 

Dapples and Rominger ^^ have shown that the long axes of quartz 
sand grains tend to parallel the direction of flow in streams, and that the 
largest ends of the grains are toward the current. Similar measurements 
should be possible along the bedding planes of some cores. Particular 
attention should be given to elongated grains. The orientation of at least 
25 randomly selected grains should be determined. 

Recent work on the directional permeability in sands ^^ suggests that 
primary orientation may be important in primary production of oil, in 
secondary-recovery operations, and in working out the paleogeography and 
geologic history of some sands. Much basic work on deposits of known 
environment is needed. For the finer sands and silts and for the lime- 
stones, dolomites, and other crystalline sediments petrofabric analysis 
will be needed. 

Composition. 

Although more than 100 minerals have been identified in sedimentary 
rocks, only about 20 minerals or families of minerals commonly are 

Dapples, E. C, and Rominger, J. F., Orientation Analysis of Fine-Grained Clastic Sediments: 

A Report of Progress: Jour. Geology, vol. 53, no. 4, pp. 246-261, July 1945. 

' Johnson, W. E., and Hughes, R. V., Directional Permeability Measurements and Their Significance: 

Producers Monthly, vol. 13, no. 1, pp. 17-25, Nov. 194S. 



126 Subsurface Geologic Methods 

present in quantities that exceed one percent of the rock. If the distin- 
guishing properties of these minerals are learned, the mineral composition 
of most subsurface samples can be determined by megascopic or binocular 
examination. 

Ordinarily the grains in coarse clastic rocks (sands and siltstones) 
will be composed of^one or more of the following: quartz, detrital chert, 
feldspar, mica (usually muscovite) , calcite, dolomite, glauconite, and 
collophane. Fragments of sedimentary, igneous, and metamorphic rocks 
also are important constituents of some clastic sediments. The crystalline 
sedimentary rocks may contain calcite, dolomite, anhydrite, gypsum, 
halite, and chert. The chief cements in clastic and crystalline sedimentary 
rocks are quartz, chert, calcite, and dolomite. Less frequently the cements 
may be pyrite, hematite, limonite, and opal. The dominant constituents 
of some mudstones and shales are the kaolin, illite, and montmorillonite 
(bentonite) groups of clay minerals and other fine-grained, platy minerals 
like sericite and chlorite. 

Probably the best method for a beginner to learn these minerals is 
to obtain and study a series of samples in which they are present, prefer- 
ably a series that has been worked previously by a good mineral man. The 
writer knows of no table that is designed primarily for identification of 
the common minerals as they are observed in cuttings. Observations ordi- 
narily may be made on some or all of the following: 

Color — Not very diagnostic because some minerals may occur in 
several colors, and color may be given by a small amount of cement or 
matrix. The green color of glauconite, however, is distinctive. 

Hardness — Not exceptionally useful because of the small size of 
the cuttings. Hardness may be determined with a probe under the binocu- 
lar microscope. 

Grain size — Very useful. 

Grain shape — Useful. 

Reaction in acid — Very useful, particularly under the binocular 
microscope. Eff"ervescence in cold dilute hydrochloric acid separates 
calcite from other carbonates. Effervescence of powder in cold dilute 
hydrochloric acid or of fragments in hot dilute hydrochloric acid separ- 
ates other carbonates. Gypsum and anhydrite are slowly soluble in hot 
dilute hydrochloric acid but do not "effervesce. Etching for about 30 
seconds in cold dilute hydrochloric acid often brings out structures and 
textures. Etching gives dolomite a flat-gray appearance that is distinctive 
and often makes the rhombic structure visible. 

Sparkle — Usually indicates (a) quartz cement, (6) dolomite, or (c) 
mica. Mica reflections will be only from the bedding plane. 

Drilling appearance — Usually more important in indicating perme- 
ability or in differentiating beds or formations than in identifying indi- 
vidual minerals. 

Shaly parting — Useful. 



Subsurface Laboratory Methods 



127 



Slaking and swelling in water — Useful in differentiating clay-mineral 
groups. Kaolins ordinarily do not slake. Very marked slaking and swell- 
ing differentiate some bentonites from other bentonites and illites. 

Smell — Earthy smell suggests kaolin; no smell suggests illites and 
montmorillonites. 

In cores the identification of minerals is the same as in other hand 
specimens. It is possible, however, to bring out structures and textures 



LIMESTONE' 

( DOLOMITE ) 



SANDY (SILTY) 
LIMESTONE 
(DOLOMITE) 



SHALY LIMESTONE 
(DOLOMITE) 



LIMY (DOLOMITIC) 
SANDSTONE 
(SILTSTONE) 



SANDSTONE 

(SILTSTONE) 




SHALY 

SANDSTONE 

{ SILTSTONE) 



SANDY (SILTY) 
SHALE 



LIMY (DOLOMITIC) 
SHALE 



SHALE 



Figure 53. Classification of the common-sedimentary rocks. (Modified from Pirsson 

and Scliucfiert.) 

of cores by preparing a polished section that can be examined mega- 
scopically or when wet with a binocular microscope. By using a carborun- 
dum stone or carborundum powder on a glass plate, one can quickly pre- 
pare such polished sections. 

Texture 

Texture which may be defined as the intimate grain-to-grain relation- 
ships of a rock, represents the sum total of such properties as grain-size 
distribution, grain shape and roundness, fabric, pore shape, and cementa- 
tion, as distinguished from mineralogic composition. Composition, in so 
far as it controls or partly controls the other properties, may be consid- 
ered a factor in texture. 



128 



Subsurface Geologic Methods 



Texture may be indicated in part by the rock name and in part by 
modifiers of the rock name. Thus the term "breccia" implies angularity 
of the component grains, whereas the term "sand" has no roundness impli- 
cation but may be modified by such terms as "angular" or "subangular." 

Structure 

Structure is somewhat similar to texture in meaning but is applied 
to such large-scale characteristics of the rock as bedding, cross-bedding, 



CLAY 

(SERICITE a CHLORITE) 




FELDSPAR 



QUARTZ 



^AND CHERT) 
Figure 54. Classification of sandstones. (Modified from Pettijohn.) 

jointing, and folding, which ordinarily are seen best in outcrops but may 
be observed in some cores or hand specimens. Except for lamination, 
structures seldom can be identified in drill cuttings. 

Rock Types 

No standard classification of rocks has been generally accepted for 
megascopic and binocular subsurface work. A relatively simple system 
that meets the needs of most subsurface work and follows the general 
usage of many subsurface men is that shown in figure 53. Subdivision of 



Subsurface Laboratory Methods 129 

the sandy and silty groups on the basis of mineralogic composition is 
desirable in light of the recent trend that attempts to tie up tectonics and 
sedimentation. Figure 54, modified from Pettijohn,^^ is a mineralogic 
classification of sands and silts that merits consideration by subsurface 
workers. Clastic limestones, that is, those in which the grains have been 
transported to their place of deposition, should be distinguished from 
limestones in which no clastic texture can be recognized. 

To the main rock name, modifiers are added to permit full description 
of the sample. As suggested by Krynine,^^ it is possible to apply to sedi- 
ments the basic standardized descriptive sequence that has been used for 
many years for igneous rocks, namely, color, subtexture, varietal minerals 
and cement, and finally the main rock name. If necessary, terms describ- 
ing structure may follow color. Thus one could have a "gray, cross- 
bedded, fine-grained, glauconitic, dolomitic, quartz sandstone." For ease 
in plotting sample data and in picking out changes in lithology, it is con- 
venient to capitalize the main rock name and place it first, and to abbrevi- 
ate as much as possible, as "QTZ SS, gy, x-b, f-gr., glauc, dol.," for the 
example above. In commercial work it is not possible to give as complete 
descriptions as might be desirable in purely scientific research, but the 
description should be made as complete as possible in the time available. 

Porosity and Permeability 

Porosity is the percentage of total volume of a rock not occupied 
by mineral components. Pores may differ in size and may be connected 
or isolated. In magascopic or binocular examination, porosity usually 
can be seen best in clean, dry samples. 

Permeability is the fluid-transmitting capacity of a porous material. 
Permeability is not necessarily a function of porosity. Clays, for example, 
may be very porous but relatively impermeable. In megascopic or binocu- 
lar examination a rough measure of permeability may be made by ob- 
serving how rapidly a drop of water will soak into a dry fragment. The 
presence of individual sand grains or oolites, rather than clusters of such 
grains, suggests slight cementation and consequently high porosity and 
permeability. 

Subsurface samples may be examined wet or dry. Both methods 
have advantages and disadvantages and when time permits a combination 
of both wet and dry study is advisable. Study of dry samples is faster 
and permits better observation of gross texture, porosity, permeability, 
sparkling, and some color differentiations. Study of wet samples is ad- 
vantageous when the samples are not clean. Some textures and colors 
also can be seen better in wet samples than in dry samples. Calcareous 
samples that have been etched with dilute hydrochloric acid are usually 
studied when wet. It is important to indicate whether color has been 

^ Fettijohn. F. J., Sedimentary Rocks, p. 526, New York, Harper & Brothers, 1949. 
^ Krynine, P. D., op. cit. 



130 Subsurface Geologic Methods 

determined from a wet or a dry sample. For binocular examination of 
both wet and dry samples magnifications of 12 to 24 diameters are com- 
monly used. Higher magnification may be used for special purposes. 
For most low-power study, flourescent is superior to incandescent lighting. 
The percentage of various constituents is usually estimated. 

Hills ^'^ notes that there are two principal ways of describing samples: 

. . . The first of these is the interpretative system, in which the geologist 
picks out the cuttings which he believes to be representative of the formations 
penetrated and describes the entire sample as composed of this rock The rest 
of the sample is assumed to be cavings. This kind of description brings out 
formational changes and is of greatest value in areas where the various for- 
mations are of wide extent and relatively constant character, as in the Paleo- 
zoic of the Midcontinent region. In areas of rapid lateral gradation in the 
lithologic character of formations, as in the Permian Basin of west Texas, this 
method results in masking of lateral variations and misinterpretations of the 
nature of the stratigraphic column. This, of course, results in miscorrelation 
of the well logs. 

In regions of pronounced lateral gradation, it has been found that a second 
method of sample description is most satisfactory. This is the percentage de- 
scription, where the geologist describes all material in the sample, disregard- 
ing obvious foreign substances and cavings. This system, though making it 
difficult to determine formational boundaries from the sample log, shows the 
gradations of the beds and often enables one to trace a horizon through differ- 
ent sedimentary facies. 

Hills also discusses many other points on sample examination that 
could not be covered in this section. 

Converted Binocular Microscope 

Recently the writer has used two polaroid plates to convert a binocu- 
lar microscope into a low-power polarizing microscope. The maximum 
magnification that can be obtained is low, being essentially the same 
as the low power of a petrographic microscope. This permits study of 
sand but not of silt and clay. The writer first tried the conversion by 
mounting sand grains in clove oil on one lens of a pair of clip-on polaroid 
sunglasses, placing the other lens above and at right angles to the first, 
and fastening the two lenses in place with drafting tape. Light was trans- 
mitted from below the glass stage of the microscope, and the grains were 
moved by pressing on the upper lens. In fine and medium sands, quartz 
grains, which are single crystals, were differentiated from chert grains 
and rock fragments, which are composed of many smaller crystals. Micro- 
cline and plagioclase feldspars could be recognized by their twinning. 
The high birefringence and relief of carbonates separated them from 
quartz, feldspar, and chert. It should be possible to differentiate anhy- 
drite, which has strong birefringence (0.044) and an index of refraction 
higher than clove oil, from gypsum, which has weak birefringence (0.010 
— quartz is 0.009) and an index of refraction lower than clove oil. 

« Hills, J. M., op. cit. 



Subsurface Laboratory Methods 131 

The color of the sunglasses was a disadvantage. It should be possi- 
ble to obtain two nearly colorless polaroid discs. Place one on the micro- 
scopic stage, mount the grains in oil on a glass microscope slide above it, 
and mount the second polaroid disc on a cardboard support that could 
be slipped in above the slide as needed. With this arrangement, however, 
rotation of the grains might be difiicult. 

Heavy Minerals 
General 

Heavy minerals are minerals of high specific gravity (2.86 to 2.96), 
which occur in minor amounts in all sands and sandy limestones. Even 
though present in small amounts, such heavy minerals as tourmaline, 
zircon, hornblende, and staurolite may be exceedingly useful in correlat- 
ing sands, outlining petrographic provinces, indicating sources and past 
history of the source material and helping to decipher geologic history. 
To facilitate their examination, heavy minerals are separated from the 
quartz and other light minerals with which they are associated. 

The study of heavy minerals has the following disadvantages: (1) 
The preparation and study of the samples are time-consuming, and conse- 
quently the costs are relatively high. (2) The ability to use a petrographic 
microscope is necessary. (3) An understanding of principles that govern 
the size distribution and post-depositional modification of heavies is neces- 
sary for the correct interpretation of the results. A lack of such under- 
standing has been partly responsible for the present low opinion of heavy- 
mineral studies in the oil industry. 

Preparation of Sample 

The heavy minerals of the entire sample or of one or more sieve 
separates may be studied. H the sieve separates are used, particular care 
should be given in cleaning the sieves, so that contamination will be kept 
to a minimum. When marked differences in amounts or kinds of heavy 
minerals occur in different samples, it may be well to sieve and discard 
a preliminary sample. In carbonate rocks or in sandstones with carbonate 
particles or carbonate cement, the carbonate should be removed prior to 
sieving by boiling in dilute hydrochloric acid. 

Indurated rocks must be broken down to free the heavy-mineral grains. 
A test sample of 5 to 100 grams may be soaked in water overnight and 
then, after the excess water has been removed by siphoning or decantation, 
the sample is boiled for ten minutes in 3 to 4. normal hydrochloric acid to 
remove carbonates and iron oxides. The sample is transferred to a 1,000-cc. 
beaker, and water is added to fill the beaker. After being stirred, the mix- 
ture is allowed to settle one minute, and the upper 800 cc. of water and 
sediment suspension is siphoned off. This washing process is repeated until 
the water is clear after the one-minute settling period. This procedure re- 
moves the fine silt and clay (less than about 0.01 mm.). 



132 Subsurface Geologic Methods 

If aggregates of particles still persist, the sample may be placed in 
the sieves and shaken by hand for about one minute. The sediment in each 
sieve is removed to a square of heavy brown paper, and the aggregates 
are broken down by roiling the end of an iron pestle gently over the ma- 
terial. At intervals the sand is resieved by hand to remove disaggregated 
particles. The completeness of disaggregation is checked by examination 
with a hand lens or binocular miscroscope. This method of disaggregation 
causes little breakage of heavy minerals, even if the sandstone is very 
quartzitic. The heavy-mineral grains, being of different composition, come 
loose like peas from a pod. The sample is recomposed and sieved, the 
procedures outlined in the discussion of that subject in this section being 
followed. 

Acetylene tetrabromide of specific gravity, about 2.93, or bromoform 
of specify gravity, about 2.86, is placed in a glass funnel, to the stem of 
which is attached a piece of rubber tube closed by a pinch clamp. The 
sand sample or sieve separate is introduced, and the mixture is stirred at 
intervals until the heavy minerals have settled into the stem of the funnel. 
The pinch clamp is opened, and the heavy fraction is washed onto a filter 
paper in a second glass funnel. After the excess heavy liquid has been 
filtered into a receptacle and returned to the stock bottle, the filter paper 
is washed several times with alcohol. The light minerals and the remain- 
ing heavy liquid are then drained onto another filter paper, the heavy 
liquid is filtered off, and the light minerals are washed with alcohol. Both 
heavy and light minerals are dried in an oven at 205° F. The alcohol- 
heavy liquid washings are saved for recovery of the heavy liquid. 

When a large number of heavy-mineral separations are to be made, 
mass-production methods can be used. The writer has employed two bat- 
teries of six separation units each, and one assistant could make as many 
as 24 separations a day. Most of these were of sand of one-eighth- to one- 
sixteenth-millimeter size, requiring about two hours of alternate stirring 
and settling to effect a satisfactory separation. For coarser sands that 
settle more rapidly, more separations a day would be possible. 

Acetylene tetrabromide and bromoform are the most commonly used 
heavy liquids. The writer prefers tetrabromide because its greater spe- 
cific gravity reduces the number of altered grains, rock fragments, car- 
bonates, micas, and chlorites in the heavy-mineral fraction. The acetylene 
tetrabromide or bromoform is reclaimed from the alcohol washings by 
shaking it with an excess of water and decanting the alcohol-water mix- 
ture. This process is repeated several times, and the acetylene tetrabro- 
mide clarified by filtering. 

Attention to two apparently minor details will save much time in 
making heavy-mineral separations. When the heavy minerals settle, most 
of them come to rest on the sloping sides of the funnels. If the suspension 
is stirred vigorously, most of these heavy minerals go into suspension and 
settle again on the funnel slopes. Consequently the suspension should be 



Subsurface Laboratory Methods 133 

stirred gently in such a way that the heavy minerals are worked down the 
slope of the funnel and into its stem. Then a vigorous stirring will free 
additional heavy minerals that are trapped with the light minerals at the 
top of the funnel. 

Selection of a proper filter paper is also important. The most porous 
paper that permits the heavy minerals to be caught and recovered should 
be used. The writer uses Whatman No. 4. A less porous paper, which 
filters more slowly, may double the time required for a heavy-mineral 
separation. 

The writer weighs the heavy-mineral fractions of most samples to 
the nearest half -milligram on an analytic balance. This weighing permits 
computation of the hydraulic ratio ^^ if that is needed. For most routine 
work, weighing the heavy-mineral separates will not be necessary. 

The heavy-mineral fractions are split to 1,000 to 1,500 grains with 
an Otto miscrosplit ^^ and mounted in Canada balsam on 1- by 2-inch glass 
microscope slides. Often more than one mount is needed, especially when 
much pyrite or barite is present in the heavy-mineral fraction. The sample 
number, size grade, and slide number are scratched on each slide with a 
diamond pencil. This provides a permanent mount, to which reference 
can be made at a later date. For easier identification, splits of the heavy- 
mineral fraction may be mounted in oils of various refractive indices. 

Mineral Identification 

Heavy minerals are usually studied with a petrographic microscope, 
using medium power (5 X ocular and an 8-mm. objective) for most work. 
The properties most useful in rapid identification of the mineral grains 
are opaqueness or translucence, appearance in reflected light (particularly 
for opaque minerals, rutile, and tourmaline), color, relief as compared 
to the mounting medium, pleochroism, inclusions and alteration products, 
crystal form, grain shape, cleavage, birefringence, extinction angle, and 
isotropic or anisotropic character. Milner's ^^ book probably is the best 
for mineral identification. Krumbein and Pettijohn"*^ and Russell ^° also 
have tables that may be used. These identification tables list many prop- 
erties of the minerals, but those given above are most useful. 

If a large number of samples is to be studied, time and effort can be 
saved by mounting the heavy-mineral suites in oils of various indices of 
refraction and identifying all species that are present. Ordinarily minerals 
that are very rare in the heavy-mineral separate may be neglected, as their 
presence or absence in any amount is a matter of chance. Then simple 
criteria can be set up by which each mineral can be recognized at sight or 

''■' Rittenhouse, Gordon. Transportation, and Deposition of Heavy Minerals: Geol. Soc. America Bull., 
vol. 54, no. 12, pp. 1725-1780, Dec. 1, 1943. 

'^ Otto, G. H., Comparative Tests of Several Methods of Sampling Heavy Mineral Concentrates : 
Jour. Sedimentary Petrology, vol. 3, no. 1, pp. 30-39, April 1933. 

*^ Milner, H. B., Sedimentary Petrography, 3d ed., 666 pp., London, Thomas Murby & Co., 1940. 

** Krumbein, W. C, anJ Pettijohn, F. J., Manual of Sedimentary Petrography, 549 pp.. New York, 
D. Appleton-Century Co., 1938. 

■** Russell, R. D., Tables for the Determination of Detrital Minerals: Nat. Research Council Co 
on Sedimentation Kept., 1940-1941, pp. 6-8, 1942. (Separate copies of tables, 50 cents.) 



-.omm. 



134 



Subsurface Geologic Methods 



by one or two rapid microscopic tests. With these criteria 90 percent 
or more of most mineral suites can be identified by observation in plane- 
polarized light. The speed of counting the grains on a slide may be 
doubled or trebled and the eyestrain greatly reduced, if an assistant re- 
cords the grain counts as they are made by the observer. 

Use of Heavy-Mineral Data 

The type of heavy-mineral data that should be obtained depends 
primarily on the objectives of the investigation, the samples that are 
available, and the heavy-mineral content of the samples. The heavy- 
mineral distribution in sandstones is closely related to the size distribu- 
tion of the light minerals in the sample and consequently the proportion 
of heavy minerals, and sometimes their presence or absence in a particular 
sample depends on the grain size of the sample. Some heavy minerals 
are removed by solution and others are deposited after deposition. These 
factors must be considered in determining the type of data to be obtained 
and how the data should be interpreted. 

Ordinarily, if different kinds of heavy minerals occur in two samples, 
for example a hornblende-epidote-ilmenite suite in one and a staurolite- 
kyanite-magnetite suite in another, the samples reflect different sources for 
the two samples. If the same kind of heavy minerals are present in two 
samples but they are present in different proportions, different sources are 
not indicated unless all of the minerals are of about the same specific 
gravity. Major differences in ratios between varieties of the same mineral 
or between minerals of about the same specific gravity usually indicate 
different sources. Even in the last case, authigenic minerals must not be 
used, and the possibility that some minerals may be removed by solution 
must be given consideration. Pettijohn^^ and Dryden and Dry den '^^ give 
the order of stability of minerals in sediments (both chemically and 
mechanically weathered) and weathered rock respectively as follows: 





Petti John 




Dryden and Dryden 


Most persistent 












Anatase (authigenic) 




Epidote 


Zircon 


100 


Muscovite 






Hornblende 


Tourmaline 


80? 


Rutile 






Andalusite 


Sillimanite 


40 


Zircon 






Topaz 


Monazite 


40 


Tourmaline 






Sphene 


Chloritoid 


20? 


Monazite 






Zoisite 


Kyanite 


7 


Garnet 






Augite 


Hornblende 


5 


Biotite 






Sillimanite 


Staurolite 


3 


Apatite 






Hypersthene 


Garnet 


1 


Ilmenite 






Diopside 


Hypersthene 


1- 


Magnetite 






Actinolite 






Staurolite 






Olivine 






Kyanite 




Lea 


ist persistent 







*^ Fettijohn, F. J., Persistence of Heavy Minerals and Geologic Age: Jour. Geology, vol. 49, no. 6, 
pp. 610-625, Aug.-Sept. 1941. 

*' Dryden, Lincoln, and Dryden, Clarissa, Comparative Rates of W eathering of Some Common Heavy 
Minerals: Jour. Sedimentary Petrography, vol. 16, pp. 91-96, 1946. 



Subsurface Laboratory Methods 135 

Large apparent differences in heavy-mineral composition may be due 
to the selective removal of certain minerals. In the example below re- 
moval of the less stable minerals would leave a very different mineral 
suite. 

Before removal After removal 

(percent) (percent) 

Hornblende 20 

Garnet 30 

Staurolite 25 

Tourmaline 15 60 

Zircon 5 20 

Rutile 5 20 

The types of minerals present in a sediment are indicative of the 
sources from which they were derived. Andalusite, kyanite, staurolite, and 
sillimanite probably indicate a metamorphic source. Ilmenite, zircon, ru- 
tile, apatite, olivine, titanite, and some varieties of tourmaline probably 
indicate an igneous source. Any heavy minerals that are very well rounded 
suggest a sedimentary source. 

Errors in Heavy-Mineral Analysis 

Heavy-mineral analysis, like other types of analyses, is subject to 
various errors that must be considered in interpreting the results. These 
errors are due to (1) the composite nature of the samples, (2) contamina- 
tion, (3) grain breakage, (4) the misidentification of the grains, and (5) 
the size of the final sample. These errors have been discussed by Ritten- 
house.^^ 

Grain Roundness 

The roundness of detrital grains may be an important criterion for 
identifying producing horizons, outlining petrographic provinces, and 
deciphering geologic history. A short discussion of roundness, giving 
particular emphasis to its interpretation, is given here. 

Roundness and sphericity (shape) are independent properties of sedi- 
ment particles and must not be confused. Roundness is a measure of the 
angularity of the corners and edges of a grain. In contrast, sphericity is a 
measure of the approach to spherical form and may be expressed roughly 
as a ratio between the length and breadth of a grain. Thus in figure 55, 
grain A has high roundness and high sphericity. Grain B has high round- 
ness but the sphericity is much lower; the length of the grain is much 
greater than its breadth. Grain C has low roundness — the corners are 
very sharp; but it is nearly equidimensional and consequently has a fairly 
high sphericity. 

*^ Rittenhouse, Gordon, Analytical Methods as Applied in Petrographic Investigations of Appalachian 
Basin: U. S. Geol. Survey Circ. 22. 20 pp., 1918. 



136 Subsurface Geologic Methods 

The three lines of four grains each in figure 55 are representative of 
three roundness classes that the writer has used for rapid roundness studies. 
Roundness varies with grain size, and consequently roundness measure- 
ments must be made on grains of the same size. Because differences and 
similarities in roundness have been found to be most significant for the 
very fine sand size, that size is recommended for initial study. 

Roundness of detrital mineral grains must be measured on the origi- 
nal grains. In many indurated or partly indurated sandstones the original 
shape of the quartz, feldspar, and carbonate grains has been modified by 
the deposition of quartz, feldspar, or carbonate on them. Consequently, 
the present shape and roundness of such grains have no significance. 
When the original grain outlines can be recognized in these sections, the 



ROUND 





O 


D.cj) 


SUBANGULAR 


C3 


^ 


Q C=) 


ANGULAR 








cO Q 



Figure 55. Representative round, subangular, and angular grains. 

roundness can be determined. When an indurated or partly indurated rock 
is crushed, the heavy minerals, being of different composition, tend to 
break out of the rock along their original boundaries. Consequently, 
heavy minerals can be used for roundness measurements in many casRS 
in which the major constituents of the rock are badly broken. 

The number of grains on which roundness, measurements should be 
made depends on the difference in roundness in the samples being studied. 
If the differences are large, fewer grains need be measured than if the dif- 
ferences are small. Probably a minimum of 25 to 50 grains should be 
measured in any case. The errors due to the size of the final sample can be 
determined from figure 56. Roundness studies are subject to the same types 
of errors as are heavy-mineral analyses. 

When quantitative measurements are made on a number of samples 
of a formation and the results are plotted on a triangular diagram, the 



Subsurface Laboratory Methods 



137 



data commonly will occupy a restricted part of the diagram. Other forma- 
tions that differ in grain roundness will be restricted to other parts of the 
diagram. Then the identity of any unknown sample can be determined. 
The roundness of tourmalines of very fine-sand size in three formations 
in Ohio is shown in figure 57. 

The rounding of tourmaline, quartz, and other hard minerals of very 




Figure 56. Curves for determining probable errors in heavy-mineral, shape, and 
roundness studies. The probable errors are expressed as percent of the total 
number of all grains; for example, with 20 percent frequency and 50 grains 
counted, the probable error is 3.8 percent.) 

fine-sand size appears to be very slow. In the Appalachian Basin the 
sands have the same roundness over thousands of square miles. There is 
no observable gradation of the type that would indicate progressive change 
in roundness due to wear as the sediment was transported along a river 
or a beach. Along certain lines the roundness changes abruptly. The 



138 Subsurface Geologic Methods 

sands on the two sides of such lines appear to have been derived from 
different sources. Thus roundness has been used to outline petrographic 
provinces. 

Because rounding of grains of very fine-sand size occurs very slowly, 
the presence of rounded grains of that size in a sediment strongly suggests 
the derivation of that sediment from a pre-existing sedimentary source. 
Also the presence of rounded and angular grains of the same mineral in 
a sample suggests derivation of the sample from two or more sources, one 



100% ROUND 




1 



100% z V V v A7 -v^iv^' ja r^'V* \/ AlOO% 

SUBANGULAR ANGULAR 

••MASSILON o SHARON ■<*. BLACK HAND 

(Salt) (Maxton) (Big Injun) 

Figure 57. Roundness of tourmalines in Massillon, Sharon, and Black Hand formations 

of Ohio (1/8-1/16 mm. size). 

of which is sedimentary. In figure 57 the Sharon is such a mixed sand. 
The use of roundness has been discussed in more detail by Rittenhouse."*^ 

Minor Minerals 

Some sands contain small percentages of grains that are distinctive 
in color or appearance. A group of sands in the same area or different 
parts of a single sand may contain such distinctive grains in different pro- 

*^ Rittenhouse, Gordon, Grain Roundness — A Valuable Geologic Tool: Am. Assoc. Petroleum Geolo- 
gists Bull., vol. 30, no. 7, pp. 1192-1197, July 1946. 



Subsurface Laboratory Methods 139 

portions. Quantitative measurement of these differences may provide use- 
ful criteria for correlation or differentation. Because distinctive grains 
may be present in a ratio to the total of 1 to 1,000, 1 to 100,000, or even 
less, the determination of the proportion by count would be very tedious 
and time-consuming. By using a good sample splitter and a binocular 
microscope, however, the relative proportions can be determined much 
more rapidly. The procedure is as follows: 

The sample is disaggregated and sieved to size the sand. Sizing is 
necessary because the proportion of distinctive grains may differ with 
size, and different ratios would occur in coarse and fine sands. The sieve 
separate is weighed to the nearest 0.01 gram on a triple-beam balance. 
Using an Otto microsplit a random sample is split out. The number of 
splits necessary to obtain the test sample is recorded; that is, the test 
sample is 1/64, 1/128, or other fraction of the original weighed-sieve 
separate. 

A 5- by 8-inch file card is folded into the shape of an M and the test 
sample is scattered as evenly as possible along the trough of the M to 
form a line of grains. The card is placed under the microscope, and the 
number of distinctive grains in the test sample is recorded as the card 
is moved across the microscope stage. Only distinctive grains are counted; 
the others are ignored. The number of distinctive grains is calculated. 
This calculation can be shown better by example than by a word descrip- 
tion; for example, the sieve separate weighs 6.02 grams, the test sample 
represents 1/256 of the sieve separate, and 17 of the distinctive grains 
were in the test sample. The number of distinctive grains per gram is 

17x256 

— — — - — =723. It should be noted that in this example each distinctive 
6.02 

grain recognized in the test sample represents 43 grains per gram. Con- 
sequently, a difference of at least 600 grains per gram more or 400 grains 
per gram less would be necessary to be considered significant. If twice as 
large a sample had been counted and twice as many distinctive grains 
had been recognized, differences of about 400 grains more or 300 grains 
less would be considered significant. 

One requisite of the distinctive grains is that they must be approxi- 
mately the same specific gravity and shape as the quartz grains. They 
cannot be flaky grains like the micas or heavy minerals like the tourma- 
lines or garnets. In the Appalachian Basin a variety of quartz that con- 
tained small wormlike inclusions of green chlorite gave useful information. 

Thin Sections 

Thin sections can be used to very good advantage in studying detrital 
mineralogy. They have the following advantages: (1) Much higher mag- 
nifications are possible than with a binocular microscope, thus permitting 
clear inspection of the smaller features of the rock. (2) The minerals 
may be identified by means of their optical properties. (3) The sections, 



140 Subsurface Geologic Methods 

being cut through the mineral grains, cement, and pores, give a view of 
these internal characteristics of the rock and their relationships to one 
another that cannot be obtained with a binocular microscope. Thin sec- 
tions have the disadvantage of requiring the use of petrographic micro- 
scope and a geologist skilled in its operation. Also the preparation of 
the sections, especially of cuttings that must be cemented together before 
being sectioned, requires some time and skill. 

INSOLUBLE RESIDUES 

H. A. IRELAND 

An insoluble residue may be defined as the material remaining after 
rock fragments have been digested in acid. Hydrochloric acid is generally 
used, but acetic acid is occasionally used if the preservation of delicate 
fossils or other structures is desired. Residues, such as shale, pyrite, gyp- 
sura, anhydrite, and glauconite, are not siliceous; therefore, the term, 
"siliceous residues" cannot be applied correctly. The chief residues are 
quartz and various types of chert, with chert the most diagnostic for 
identification and correlation, 

McQueen and Martin in 1931 published methods of preparation, 
terminology, and practical application of insoluble residues to surface 
and subsurface correlation and identification of calcareous rocks. The 
work of Martin is not known so well as that of McQueen although it is a 
significant contribution. The use of insoluble residues was not wide- 
spread prior to 1938. After 1940 rapid advances were made with the 
application of residue work to petroleum geology. The United States 
Geological Survey and many state surveys now have many publications 
based wholly or in part on insoluble residue work. Most of the residue 
work in Texas was developed independently of that of McQueen, and a 
diversity of nomenclature resulted. In 1946 Ireland called a conference 
of active workers from the central United States, which resulted in the 
publication of standardized terminology and a chart, which is published 
herein in modified form (See table 3.) 

Preparation of Residues 

Types of Samples 

The materials treated for insoluble residues are well cuttings, cores, 
and outcrop samples. The most desirable outcrop samples are channel 
samples or a composite mixture of each exposed stratum within a five-foot 
or other closed interval. Point-to-point correlation is rarely possible, 
since there is very little probability of sampling exactly the equivalent 
point some distance away. A six-inch layer outcropping within a five-foot 
interval will not represent the whole interval, and it cannot be correlated 
with the equivalent interval a mile away, which may have a six-inch layer 



Subsurface Laboratory Methods 141 

exposed a foot above or below the one in the first outcrop. Only zones or 
intervals may be correlated successfully. Outcrop samples of unweathered 
chips, without lichen, soil, or other extraneous matter are desirable. 

Oil- or water-well cuttings and cores are the most widely used mate- 
rials for residues. Cable-tool cuttings are the best samples, as they contain 
a minimum amount of caved material, and each sample represents a com- 
posite of the rock within the sampled interval. 

Rotary-tool cuttings are the most common well samples and are gen- 
erally the only type of samples available from deep wells. They are also 
the worst samples. Caving is very common because long sections of the 
drill hole are frequently open. If shale beds or loosely aggregated mate- 
rials lie above a given sample, caving may reduce the amount of indigenous 
material of the sample to such a small percentage that an insufficient 
amount of residue or none will be left after solution. Such samples may 
require the use of forceps for picking out chips of the indigenous material 
for solution. Drilling time, electric logs, and a thorough knowledge of 
the section facilitates the identification of the indigenous material. 

Well cores must be split and a fragment taken from each inch or 
short interval, and the whole mixed for the equivalent of a five-foot sam- 
ple, or a shorter interval if the lithology changes. Otherwise, inconclusive 
point-to-point correlation would be necessary. 

The observable amount of indigenous material in a sample having 
eighty to ninety percent shale caving may be increased by placing two or 
more unit volumes of the sample in acid, and, after solution, sieving out 
as many unit values less one. Thus, if three units were used, two units 
would be sieved out after solution. This will leave less than one unit 
volume, which will contain a minimum amount of caved material but 
several times more residue from the indigenous material. Large frag- 
ments of chert or other insoluble material considered indigenous may be 
picked out with forceps from the sieve and added to the residue. 

Amount of Sample 

The volume or weight of sample used to make a residue depends on 
the purpose of the study, the type of samples used, and individual judg- 
ment. Seven grams is an ample amount of sample for ordinary uses. This 
weight is an average for the volume contained in a one-dram vial, 45 by 
15 mm. The same volumes of ten different homogenous samples ranging 
from very fine to very course fragments of limestone, shale, sand, and 
chert were weighed, and the average of seven grams was determined. The 
volume-weight of seven grams reduces considerably the time for the prepa- 
ration of residues. Small samples of less-than-unit volume must be weighed 
if percentage determinations are desired. The use of a small scoop sized 
for a unit volume or a tip balance saves time. 

Many workers do not use percentages, but the percentages of residue 
are valuable in many cases for correlation and identification of beds. 



142 Subsurface Geologic Methods 

Samples from rotary tools can rarely be used satisfactorily for percentage 
determinations unless caving intervals have been cased. 

Siliceous limestone, tripolitic or cotton chert, and calcareous shale 
may lose up to half their weight but retain unit volume after solution. 
Such samples should be weighed before and after acidification, if the 
percentage of residue compared to the original volume is needed. If the 
volume of the vial is used as the unit of the original sample, the percentage 
of nonporous residues may be scaled or observed through the glass vial. 
Pure limestone or dolomite samples from cores or outcrops may leave 
only a few grains of residue, and it may be necessary to use two, three, or 
even five units of the original sample to obtain sufficient residue for 
examination and determination. 

Solution of Samples 

Samples are generally dissolved in commercial hydrochloric (muri- 
atic) acid. It is inexpensive, easily obtained, and effective. The acid 
should be diluted with water to at least fifty percent but to no less than 
ten percent. Warming will hasten the reaction, but undesirable precipi- 
tates may form. Many complex reactions occur between caved material, 
constituents of the indigenous material, and the impurities in the muriatic 
acid. Iron, gypsum, and other precipitates frequently coat, stain, and 
contaminate many types of residues. Many samples will not dry clean 
if left in the spent acid and precipitates longer than six to eight hours. 

Chemically pure (CP) hydrochloric acid has advantages for special 
work where outcrop samples are used, where precipitates or impurities 
are undesirable, or when solution is extended over several days. Acetic 
acid is best for liberation of delicate, fragile, lacy material or for micro- 
scopic organisms. Delicate residues may be preserved by using very dilute 
hydrochloric acid, but the time required for solution is lengthened. 

Beakers are the best receptacles for solution of the samples. They 
are preferred because the lip facilitates washing and decanting, residues 
may be easily removed, and a glazed spot is provided on the side for 
identification of each sample. Molded tumblers or other cheap glassware 
may be used, but the breakage due to heat while drying samples equals or 
exceeds the greater original cost of heat-resistant glassware. 

The procedure for residue preparation is simple. Samples of unit 
quantity are placed in a glass receptacle properly identified on a slip of 
paper under a pyrex dish or by any consistent regular arrangement. Sam- 
ples are then digested in acid, washed, dried, labelled, and stored for 
examination. The use of several stainless-steel trays, or other type of tray, 
holding forty to fifty beakers, facilitates the bulk movement of samples 
to the hood for acid application, washing, drying, or other operations 
involving the handling of large numbers of samples. 

The first application of acid should be small to prevent foaming 
caused by the rapid effervescence of powder and fine material. The foam- 



Subsurface Laboratory Methods 143 

ing may easily cause the overflow and loss of considerable material. A few 
minutes after the initial application, additional acid may be added, but 
only experience will tell how much, generally not more than one-third to 
one-half the capacity of the receptacle. After several hours of digestion 
the samples should be washed once or twice to remove spent acid, pre- 
cipitates, and undesirable material. The second application of acid will 
generally complete the digestion, although one application may be suf- 
ficient. Small applications of acid will digest samples which obviously are 
chert, sand, or shale. Incomplete digestion will leave dolomite pellets with 
rough, jagged surfaces and rounded pellets of limestone. When samples 
are incompletely dissolved, individual euhedral dolomite rhombs may be 
a large part of the residue. Final washing should be thorough to remove 
all traces of acid and prevent scum, caking, or coating on the residues. 

Clay and fine silt are generally decanted in routine work. Little or no 
work has been done with the fine residues, and their value for correla- 
tion and identification is yet to be determined. Only outcrop or core sam- 
ples can be used for study of clay and silt residues, because caving and 
other contamination of well samples obscure diagnostic features and 
makes uncertain the identification of indigenous fine clastic material. 

Residues may be dried in an oven, on a hot plate, or on a sand bath. 
Dry residues are brushed into a pan or funnel for transfer into glass vials, 
which may be labelled on the cap, cork, or a paper sticker. Permanent 
storage requires a painted label or glazed surface on the side of the vial, 
because silverfish enjoy eating the glue from stickers. One-dram vials 
hold ample residue for study and require very small storage space. Trays, 
drawers, original vial boxes, or special boxes are suggested methods of 
storage. 

Description of Residues 

The most common insoluble residues are chert, chalcedony, dissem- 
inated silica, clastic and crystalline quartz, aluminous matter, and replaced 
fossils. Anhydrite, gypsum, feldspar, glauconite, hematite, pyrite, fluorite, 
and sphalerite are the most common minerals, but other insoluble miner- 
als are found. 

Table 3 is a modified arrangement of the original chart published 
with the paper on standardized terminology. ^° The terminology is based 
on description rather than genesis of the residues because genesis of many 
constituents is unknown, vague, or controversial. Many possible types of 
residues are given a place in the table, although their existence has not 
been confirmed. Each term is clear-cut and restrictive, and within certain 
limits a residue fragment may be pigeon-holed. It should be emphasized 
that types of residues grade into other types, and, as some specific frag- 
ments may not be easily placed, workers may place a fragment under a 
different but related type in the classification. 



'"Ireland, H. A., et al.. Terminology for Insoluble Residues: Am. Assoc. Petroleum Geologists 
Bull., vol. 31, no. 8, pp. 1479-1490, Aug. 1947. 



144 



Subsurface GEOLOcrc Methods 



TABLE 3 
Chart for Insoluble Residues 



Quartz 



Euhedral 



Subhedral 



Ap.hedral 



Loose 
Drusy 
Granulated 

I 



Unmodified 
Lacy 
Drusy 
Dolomorphic 



Fine 



Coarse 



Dolomoldic Oomoldic 

\ / 

Skeletal 

Abundant 

Scattered 



Loose 

Aggregated 

Granulated 



Oolitic 

I . • 
Concentric 
Radiate 
Sand-centered' 
Massive 
Clustered 
Free 
Drusy 



Chert 



Smooth 



Chalcedonic Ordinary 



Porcelaneous 



Granular 



Chalky 



Fine 



Coarse 



Unmodified Dolomoldic Oomoldic 



Lacy 

Drusy 

Dolomorphic 



\ / 

Skeletal 

Abundant 

Scattered 



Oolitic 


Granulated 


1 


Sandy 


Concentric 


Silty 


Radiate 


Banded 


Sand-centered 


Spicular 


Massive 


Fossiliferous 


Clustered 




Free 





Subsurface Laboratory Methods 

TABLE 3— Continued 
Argillaceous material 



145 



Spongelike 



Clay 
_J 



naky 

I 



Massive 



\/ 



Shale 



Unmodified Dolomoldic Oomoldic Oolitic 

Lacy 

Dolomorphic 

Skeletal Concentric 

Abundant Radiate 



Sandy 

Silly 

Fossiliferous 

Glauconitic 

Pyritic 



Scattered 



Sand-centered Micaceous 



Smooth 

I 
Flaky 
Waxy 
Laminated 



Massive 



Other minerals 



Silt 



Arenaceous material 
I 



Loose 



Consolidated 



Poorly 



Well 



I ~1 \ 1 

Fossiliferous Dolomoldic Oomoldic Oolitic 

Sandy \ / I 

Quartzose \ / I 

Glauconitic Abundant Concentric 

Pyritic Scattered Radiate 
Micaceous 
Other minerals 



Sand 



Loose 



1 

Consolidated 



Poorly 



Well 

_J 



Rounded Angular 

Subrounded Regenerated 

I 
Frosted 
Polished 



Sand-centered Etched 
Massive 



Massive 



Anhydrite 



Fibrous 



Subhedral 



Fine, granular 

I ^ 

Subhedral Anhedral 



Coarse aggregates 



Anhedral 



Massive 



Gypsum 



Fibrous 



Selenitic 



Unclassified Accessory Residues 

Sulphur, pyrite, marcasite, sphalerite, millerite, magnetite, hematite, limonite, 
feldspar, muscovite, biotite, chlorite, glauconite, barite, celestite, other insoluble min- 
erals, fossils, pellets, beekite. 



146 Subsurface Geologic Methods 

Terminology for Insoluble Residues 

Definitions of special terms as agreed upon by the Residue Confer- 
ence of 1946 are given below in alphabetic order. 

Abundant dolomolds or oomolds: See "dolomoldic." 
Anhedral: No crystal form developed. 

Beekite: Botryoidal, subspherical, or discoid accretions of opaque silica 
replacing organic matter, generally white. 

Chalcedonic: Transparent to translucent; smoky; milky; waxy to greasy; 
may be any color, generally buff or blue-gray; may be finely mottled. 

Chalky: Uneven or rough fracture surface; commonly dull or earthy; soft 
to hard; may be finely porous; essentially uniform composition; resembles 
chalk or tripolite. (Formerly referred to as "dead" or "cotton chert." This in- 
cludes dull, unglazed porcelaneous material which grades into glazed por- 
celaneous material of smooth chert.) 

Chert: Cryptocrystalline varieties of quartz, regardless of color; composed 
mainly of petrographically microscopic fibers of chalcedony and/or quartz 
particles whose outlines range from easily resolvable to nonresolvable with 
binocular microscope at magnifications ordinarily used by geologists. Particles, 
rarely exceed 0.5 mm. in diameter. 
Clay: Fine material of clay size. 
Clustered: See "oolith." 
Concentric: See "oolith." 

Dolomold: Rhombohedral cavities in an insoluble residue. (Generally due 
to the solution of euhedral dolomite or calcite crystals.) 
Dolomoldic: Containing dolomolds. 

Skeletal with dolomolds: Residues with rhombohedral open- 
ings in which the constituent material comprises less than 25 
percent of the volume of the fragment. Openings vary from micro- 
scopic to megascopic. 

Abundant dolomolds: Residues with rhombohedral openings 
with the constituent material comprising from 25 to 75 percent 
of the volume of the fragment. Openings vary from microscopic 
to megascopic. 

Scattered dolomolds: Residues having rhombohedral open- 
ings in which constituent material comprises more than 75 per- 
cent of the volume of the fragment. Openings vary from micro- 
scopic to megascopic. 
Dolomorphic: Used for describing residues where there has been replace- 
ment or alteration of dolomite or calcite by an insoluble mineral which assumes 
the crystal form of the soluble mineral, thus filling a dolomoldic cavity. 
Drusy: Clusters or aggregates of crystals, generally incrustations. 
Euhedral: Doubly terminated crystals; unattached. 
Free: See "oolith." 

Granular: Chert; compact, homogenous; composed of distinguishable re- 
latively uniform-size grains, granules, or druses; uneven or rough fracture sur- 
face; dull to glimmering luster; hard to soft; may appear saccharoidal. (This 
type is frequently referred to as "crystalline.") 

Granulated: Grains or granules partly cemented or loosely aggregated; 
saccharoidal; grades from angular to drusy; fine to coarse; particles rarely 
larger than 0.5 mm. in diameter. 

Lacy: Residues with irregular openings in which the constituent material 
comprises less than 25 percent of the volume of the fragment. 

Massive: See "oolith." Used also to include fine or coarse granular 
anhydrite or gypsum. 



Subsurface Laboratory Methods 147 

Mottled: Residue fragments with two or more colors or different material 
interspersed and irregularly shaped with the boundaries between either sharp 
or gradational; often appears flocculated; grades into speckled residue. 
Oolite: Composed of an aggregation of ooliths. 

Oolith: Spheroidal bodies with nucleus or central mass enclosed by one 
or more surrounding layers of the same or different material; may be any color 
and of many kinds of material, generally less than 1.0 mm. in diameter. Those 
over 2.0 mm. are pisoliths. 

Concentric: Peripheral layers around a small, undetermined 
nucleus. 

Clustered: Attached ooliths without solid matrix. 
Drusy: Oolith covered with subhedral quartz; may be free 
or clustered. 

Free: Unattached oolith. 

Massive : Interior of granular, smooth, or chalk-textured mate- 
rial comprising nearly the entire mass of the spheroid. 

Radiate: Fibers radiating from small or large nucleus; may 
have several peripheral layers. 

Sand-centered: Nucleus, a quartz sand grain. 
Oomold: Spheroidal opening representing the former pres- 
ence of ooliths. 
Oomoldic: Containing oomolds. 

Skeletal with oomolds: Same definition as for "dolomoldic." 
Abundant oomolds: Same definition as for "dolomoldic." 
Scattered oomolds: Same definition as for "dolomoldic." 
Ordinary: smooth chert with even fracture surface; all colors, chiefly 
white, gray, or brown; may be mottled; approaches opaque; generally homo- 
geneous, but may have slight evidence of granularity or crystallinity ; grades 
into chalcedonic or granular chert. 

Porcelaneous: Chert with smooth fracture surface; hard; opaque to sub- 
translucent; typically china-white resembling chinaware or glazed porcelain; 
grades to chalky. 

Pseudoolithic: Rounded pellets with no peripheral layers or sharp dis- 
tinction between pellets and matrix. 

Quartz: Clear, colorless quartz; not detrital. 
Radiate: See oolith." 

Regenerated: Used in reference to quartz sand grains with secondary re- 
growth of crystal faces oriented with the original axis of the grain. 

Rounded: Spheroidal or ellipsoidal sand grains, coarse to fine, may be 
polished, frosted, or etched. 

Sand: Grains of sand size, chiefly quartz, but may be composed entirely or 
partly of other minerals. 

Sand-centered: See "oolith." 
Scattered: See "dolomoldic" and "oomoldic." 

Silt: Grains of silt size, chiefly quartz, but may be composed entirely or 
partly of other minerals. 

Skeletal: See "dolomoldic" and "oomoldic." 

Smooth: Major type of chert with conchoidal to even fracture; surface 
devoid of roughness; may be botryoidal; homogeneous; no distinctive struc- 
ture, crystallinity, or granularity. 

Spicular: Containing inclusions of sponge spicules. Free spicules have 
been noted. 

Speckled : Disseminated fine spots of color or material different from that of 
the matrix and having relatively sharp boundaries. 



148 Subsurface Geologic Methods 

Subhedral: Crystal forms partly developed; may be loose, drusy, or granu- 
lated. 

Subrounded: Polygonal grains or fragments but with well-rounded edges 
and corners. 

Unmodified: Residue uniform with no modifying characteristics. 

The most common residues are chert and sand, with chert rated as 
the most diagnostic. Texture, color, transparency, luster, and crystallinity 
are the chief factors for the differentiation of chert. Inclusions and modi- 
fying characteristics are secondary factors. Chalcedonic and ordinary chert 
are the most abundant of the smooth cherts. The term "granular chert" 
is applied to obviously crystalline chert or that with observable grains. 
Smooth and granular cherts grade into each other and into chalky chert. 
The chalky types are those of which the original internal structure and 
filled interstices have been affected by weathering and probably by cir- 
culating water. Tripolitic chert when placed in acid leaves a very fine, 
porous, chalky chert because of the solution of disseminated calcium car^ 
bonate. All the cherts may be dolomoldic, the dolomolds ranging in type 
from scattered to skeletal and in size from fine to very large. 

The color of chert is an important diagnostic feature. It is prevalently 
colorless, white, gray, tan, and brown, but all colors are found. Many 
residues from beds in Missouri, Kansas, Oklahoma, and Texas have sudden 
color changes which mark boundaries of zones or formations. The smooth, 
brown chert of the Lower Devonian in west Texas is diflScult to differen- 
tiate from that in the Upper Ordovician, and drilling to an underlying 
boundary is necessary in many places for positive identification. 

Organisms may be replaced by silica or other insoluble matter and 
may be identified in the residues, especially small forms and Foraminifera, 
which generally are not broken by the drill. Molds of organisms are com- 
mon where soluble shells or fragments have been imbedded in an insolu- 
ble matrix. Beekite occurs most commonly in replaced megascopic fossils 
found in outcrop samples. 

Quartz may be euhedral and authigenic or subhedral and anhedral 
from veins, cavity filling, or interstitial openings. Quartz sand of various 
types from rounded to angular may be found as scattered inclusions or 
as a dominant feature in a sandy calcareous rock. Secondary enlargement 
or regrowth of quartz crystals around sand grains is a common occurrence 
in the Lower Paleozoic. The crystal growth in some sandstones is dis- 
torted and interlocked with adjacent grains in such a manner that a tight, 
nonporous formation results. Feldspar, mica, glauconite, and other min- 
erals are common as residue constituents of sandstone, although quartz 
is the chief constituent. Calcareous material interstitially mixed with very 
fine quartz in silt and clay sizes results in a very fine porous residue. 

Glauconite is abundant in sands and is scattered throughout many cal- 
careous beds. It is a good marker for many beds in the Paleozoic, chiefly 
in the Mississippian, Middle Devonian, Middle and Lower Silurian, and 



Subsurface Laboratory Methods 149 

Upper Cambrian. Few of the lower Ordovician Beekmantown beds have 
glauconite, and the appearance of glauconite generally marks the top of 
the Cambrian. 

Pyrite is a common insoluble residue seen as small to large, euhedral 
crystals in limestone, dolomite, and shale. It also occurs spongelike, dis- 
seminated, and in veins and cavities. Pyrite has little value as a diag- 
nostic residue, but it has a secondary value as an inclusion in chert or 
shale. When pyrite occurs in abundance it may serve as a marker bed 
and often identifies a zone of circulating water or an unconformity. 

Interstitial spaces due to primary or secondary permeability, altera- 
tion, or replacement in calcareous rocks may become filled with silica, 
pyrite, or other insoluble material. Solution of the matrix leaves fragile, 
lacy networks that are generally destroyed by acid effervescence and wash- 
ing. These residues are the extreme upper limit of skeletal dolomolds, 
pyrimolds, and oomolds. Residues from veins or fractures are curved or 
tabular flakes. Vein fillers or cement for brecciated residues include gil- 
sonite, silica, pyrite, and sphalerite. 

Siliceous limestones have residues that are generally earthy, finely 
porous, and dark-colored. These residues are especially noteworthy be- 
cause examination of such samples before solution gives no clue to the 
type of residue. The residues from siliceous limestone also appear to be 
100-percent insoluble by volume, but they may be 50-percent insoluble by 
weight, owing to the removal of the interstitial lime. 

Siliceous oolites are common and may be found free, clustered, or 
in a matrix. An oolite, to be identified as such, must have a nucleus and 
at least one concentric layer or shell. Nuclei may range in size from very 
minute to one occupying nearly all of the interior mass. Most ooliths have 
several shells. Ooliths are classified according to the interior structure as 
concentric, massive, radiate, or sand-centered. Clustered or free ooliths 
may be frosted with a crust or minute drusy quartz or may have a smooth, 
siliceous shell. Silica may replace calcareous ooliths and cause them 
to be presjerved as residues. Ooliths have many colors and frequently 
occur embedded in different-colored matrices. All types of chert have 
ooliths, although in chalky chert they are rare. 

Cherjt in many cases has included sand grains, which may be con- 
fused with ooliths. Shells are absent, however, and the clear quartz of 
the said grain may be observed. 

Pseudoolites or "shadow oolites" resemble oolites and may resemble 
included quartz sand grains. The boundary between the matrix and the 
oolith is indistinct, however, and the central portion, which cannot be 
identified as quartz is only a shade lighter or darker than the other por- 
tions. Pseudoolites may be ooliths or sand grains that have been resorbed, 
thus destroying any formerly existing boundaries. 

Dolomolds occur chiefly in chert residues from dolomites, rarely in 
chert from limestone. Dolomolds are common in shale residues and are 



150 Subsurface Geologic Methods 

present in some pyrite and glauconite residues. Natural dolomolds result- 
ing from weathering are common on certain types of outcrop samples. In 
dolomite the dolomolds are assumed to be the impressions from dissolved 
dolomite rhombs, but in shale the cavities are likely to be a result of dis- 
solved interstitial calcite. Disseminated abundant fine dolomite or calcite 
crystals in chert, silt, or shale will leave a very finely porous residue, too 
fine to be observed except under high magnifications. The residue of a 
sample with large quantities of dolomite rhombs will have an intersecting 
lacework of fragile skeletal dolomolds, while a sample with a few rhombs 
will leave scattered dolomolds in the insoluble matrix. Dolomolds may be 
large or small, but generally all in any one fragment will be essentially the 
same size. 

Use of Insoluble Residues 

The study of insoluble residues is a supplement to and not a substitute 
for lithologic sample examination. The cost of preparing and filing resi- 
dues and the longer time necessary for the more detailed and careful exam- 
ination of them are factors that must be considered. The mass character- 
istics of the major constituents of insoluble residues generally have enough 
similarity horizontally and vary enough vertically to serve for identification 
and correlation of lithologic units within a thick section of calcareous rock. 

Lithologic similarities of thick sections of nonfossiliferous calcareous 
rocks prevent their subdivision into thinner zones for more-detailed corre- 
lation and identification and structural mapping. Insoluble constituents 
having diagnostic characteristics may be obscured by the volume of the 
fragments in a lithologic sample and by being embedded in a solid 
matrix. These constituents are liberated, concentrated, and exposed by 
solution of the matrix. Diagnostic material such as Foraminifera, some 
types of chert, dolomolds, disseminated pyrite, fossil replacements, euhe- 
dral crystals, mineral or clastic inclusions, and silt aggregates are not 
observed or recognized until they become residues. 

Residues reflect clastic conditions, sea-bottom environment, current 
action, and adjacent land-mass conditions, which may supply various 
types of source materials. These factors may change independently over 
short or long periods of time. If the source of material and the conditions 
of deposition or precipitation of calcareous matter remain fairly constant 
for a long time, no significant lithologic variations would result that 
might serve to identify a stratum. A slight change involving the source, 
type, or amount of clastic furnished to a lime-depositing environment 
might not affect greatly the lithologic appearance of a sediment, but 
such material when left as a residue would be diagnostic and serve for 
correlation and identification. The amount of silica, iron, or salts in the 
sedimentary basin might change and give pyrite, siliceous limestone, vari- 
ous types of chert, and other minerals or constituents of diagnostic value; 



Subsurface Laboratory Methods 151 

all of which might be independent of clastic material or changes in land- 
mass conditions or source material. 

Circulating water and replacement and alteration of constituents 
before and after lithification would change the original residues. These 
changes, if of sufficient magnitude, might be observed in a lithologic exam- 
ination of samples, but only the study of residues would show the small 
changes that might be useful in a detailed subdivision of beds. Correla- 
tion using residues of secondary origin could only be used locally or as 
far as the effect of the modifying conditions could be traced. 

Correlations for distances greater than fifty miles are risky, unless 
some significant wide-range constituent can be determined, because the 
residues will change as the sedimentary environment changes. Obviously 
correlation using any specific zone of residue types would be less reliable 
in a basinward or landward direction than laterally in a direction at 
right angles. 

Correlation of individual thin beds may be difficult because of lateral 
and vertical changes of the sedimentary-environment time. The subdi- 
vision of a thick calcareous section and the inclusion of nondiagnostic 
thin beds into zones make correlation possible. Identification of the zones 
is based on such factors as sequence of beds, position in the section, per- 
centage of residue, association of types of residues, and dominant charac- 
teristics with chief reliance on dominant characteristics. A distinctly 
significant residue may identify certain zones, although other residue 
constituents may be present, and even though the diagnostic residue is 
not the dominant one. An assemblage of residue constituents often deter- 
mines the correlation or identification just as an assemblage of fossils 
serves for determination. Both microscopic and macroscopic fossils re- 
placed with insoluble material are valuable in some zones. 

Positive identification of some subdivisions is difficult with only a 
few samples, unless a significant break or change in residue occurs within 
the interval examined. For example, assume that a limestone 1,400 feet 
thick is divided into six zones having intervals of 350, 100, 200, 400, 300, 
and 50 feet. If only ten 5-foot samples were available from zone 1 at the 
top, it would be difficult or impossible to identify their positions in the 
zone, although the zone itself could be identified. If the samples over- 
lapped into zone 2, then the boundary could be recognized, and it could 
be stated than 300 feet or more of zone 1 was absent. 

Many cherts are alike in color and texture, and similar cherts in 
two different zones would prevent identification, unless an associated 
residue was diagnostic or a zone boundary was passed. The similarity 
of the brown, smooth chert in the Lower Devonian and the Upper Ordo- 
vician in west Texas has been mentioned previously. If a chert in zone 1 
was similar to a chert in zone 4 in the section postulated in the last para- 
graph, the two zones could easily be confused. If the set of ten samples 



152 Subsurface Geologic Methods 

was identified as belonging to zone 1, but actually zones 1, 2, 3, and part 
of 4 had been eroded, an error of at least 650 feet in correlation would 
result. The correct identification of zone 4 would show a structural upfold. 
If the samples were identified as zone 4 and the producing bed was zone 2, 
the absence of zone 2 would be concluded and deeper drilling prevented. 

The foregoing discussion shows the necessity of having some associ- 
ated diagnostic residue or a zone boundary included in the sample inter- 
val for positive identification. Knowledge of the similarity of two zones 
would call for careful drilling and a postponement of identification until 
the underlying zone was encountered. With lithologic examination no 
zones could be identified. 

Pyrite, regenerated sand grains, a sandy chert or sandy zone, a shale 
break, or a detritus often present clews to a formational change, which in 
some cases can be confirmed by other evidence. 

The use of residues is not restricted to the laboratory. A microscope, 
a jug of acid, and a half-dozen beakers may be carried to the field. Water 
from a drilling well may be used for washing, and heat from an automo- 
bile-engine head or a drilling well will dry the samples for examination 
on location. Obviously, a geologist attempting such work must be familiar 
with residue zones and sequences, as the necessary samples for compari- 
son would not likely be available. 

New workers with residues should be well aware that successful cor- 
relation by residues comes only after a thorough knowledge of residue 
types, principles of secondary replacement, and facies changes and the 
examination of many samples. Experience with residue material is pre- 
requisite to the successful correlation and identification of beds. Of 
course, the foregoing statement is true for lithologic examination, but an 
inexperienced geologist can soon learn the surficial characteristics of 
rock fragments and correctly correlate, but he would find it difficult to 
correlate with residues without experience or the supervision of one 
experienced in residue work. 

The use of residues for correlation has been successful in the thick 
calcareous sections of most Paleozoic rocks but has had little success in 
the thick Permian section of west Texas and New Mexico. Residue work 
has been especially useful in subsurface work and petroleum geology in 
Texas, Oklahoma, Kansas, Missouri, and Illinois and has contributed 
much to geologic science in the states between the Appalachian and the 
Rocky Mountains. The beds receiving the most attention have been Upper 
Cambrian and Lower Ordovician, but Silurian, Devonian, and Mississip- 
pian beds have been extensively studied. 

The space allotted here would be inadequate to give worth-while 
descriptions of the subdivisions of the thick sections of calcareous rocks 
in the various parts of the United States. Anyone concerned would profit 
more to confer with workers familiar with local areas and sections. 



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154 Subsurface Geologic Methods 

The extensive and successful application of residues to petroleum 
geology proves the value of residue studies, but few petroleum geologists 
have published results. The Insoluble Residue Library of Midland, Texas, 
is financed and operated by nineteen companies, which employ specialists 
for residue examination or subscribe to a special service furnished by the 
Midland Residue Research Laboratory. The Missouri Geological Survey 
uses insoluble residues as a standard procedure for the correlation of 
formations younger than Pennsylvanian. Its collection of residue samples 
is probably the largest and the finest collection in the world. The state 
geological surveys of Illinois, Kansas, Missouri, Oklahoma, and Texas and 
the United States Geological Survey have utilized the study of residues as 
a regular part of their programs for subsurface work. 

Plotting Residue Data and Descriptions 

Many methods of plotting residue data have been devised for indi- 
vidual needs and purposes. Three will be discussed here. The writer 
uses a method called the "constituent-percentage method," which is illus- 
trated in figure 58. The data and description are plotted on a strip log 
100 feet to the inch printed with a grid ruling. This scale allows the 
comparison of residue logs with standard oil-well logs or sections. Other 
intervals may be used according to the need and desire for detailed de- 
scription. 

Column 2 shows the percentage of residue in relation to the original 
sample, and column 3 shows the lithology. The percentage of each con- 
stituent in reference to the total residue is plotted in column 4. Thus the 
percentages of the constituents from a ten-percent residue of the original 
sample will be shown in the same lateral space as the percentages from 
a ninety-percent residue. Color and symbols with superscrips and over- 
prints in ink over the colored background in column 4 describe and dis- 
tinguish the constituents. The most specific information for correlation 
work appears in this column. Lines representing the color of the cherts 
are placed in column 7, the color being the same as the actual color of 
the chert, except that white chert is designated by green. 

The percentage-percentage method is a second method of plotting. 
By this method the percentage of each constituent is plotted in proportion 
to its percentage of the original sample as shown in figure 59. Super- 
scripts and overprints in ink and color similar to the first method are 
used for differentiation of the constituents. An expanded scale is required 
and percentages over 75 are eliminated. 

Residues from all types of samples may be plotted satisfactorily by 
this method except rotary-tool samples having considerable cavings. The 
caved material in rotary-tool samples hinders accurate judgment of the 
percentage of any one constituent in relation to the indigenous portion 



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156 Subsurface Geologic Methods 

of the original sample. In all samples the data for percentage-percentage 
plotting must be calculated or a table employed. A major disadvantage 
of the method is that a constituent which is ten percent of a residue, 
which in turn is ten percent of the original sample, requires the plotting 
of a 0.01-percent space on the log. Such a small space is difficult to 
plot as well as to identify in later study and correlation work. Though 
a ten-percent residue is ample for determination, many residues are less 
than five percent. Small but significant and diagnostic constituents would 
be obscure and very difficult to differentiate on a log. This method of 
plotting has an advantage in showing the percentage relations of the 
constituents at a glance, eliminating the examination of both the per- 
centage and constituent columns of the first method discussed. 

The third method of assembling data is a tabulation. A number of 
columns, headed by the names of significant types of residues, allow space 
for tabulating the percentage of each constituent and for symbols with 
superscripts or abbreviations added as modifying descriptions. This 
method is not suitable for correlation work and cannot be used in con- 
junction with the standard scale of plotted logs. It is useful only for 
tabulating data for the use of log plotters or others making strip logs or 
for consultation when detailed information not amenable to log plotting 
is needed. 

The first method has been found to be the most satisfactory, and it is 
recommended, although several versions of the three have been used, and 
other combinations may be devised. It is most desirable for all workers 
to use a standard set of symbols, superscripts, and overprints. This applies 
especially to new workers entering the field of insoluble residues. Work- 
ers could then examine, discuss, interpret, and publish insoluble-residue 
correlations and identifications with a common background. Many workers 
will find it difficult or unwise to change systems of graphic description, 
because consistence with former usage is necessary where logs or records 
are involved. 

The sets of symbols and overprints given here are recommended for 
standardized use. They are essentially those used by the Missouri Geolog- 
ical Survey. Certain modifications, combinations, and additions make the 
symbols conform to the recently standardized terminology. Color was 
used formerly by the writer to indicate the observed color of the chert; 
but color is now used to indicate the type of chert, and chert colors are 
indicated in a separate column. 

Hendricks ^"^ uses a set of letters, lines, bars, or graphics without 
color. If the percentage of the residues, the color of the residues, or the 
percentage of the constituents is not desired or necessary, the set of sym- 
bols is adaptable. Many of them may be made with a standard typewriter. 

^"'^ Hendricks, Leo, Subsurface Divisions of the Ellenburger in North-Central Texas: Texas Univ. 
Bur. Econ. Geology Bull. 3945, pp. 923-968, 1940. 



Subsurface Laboratory Methods 157 

PETROFABRIC ANALYSIS 

WARREN R. WAGNER 

Although petrofabrics has not been widely used, it is a valuable 
geologic tool that may be applied with marked success in deciphering 
complicated structural conditions (surface and subsurface). The detail 
to which it is carried depends upon the nature of the problem; its applica- 
tion, however, may produce results on broad reconnaissance studies or on 
large-scale detailed problems. The greatest value is probably obtained in 
the latter. 

The idea that the petrofabric data for an entire area may be obtained 
from one thin section and presented by making one or two orientation 
(petrofabric) diagrams seems to be prevalent with some geologists. One 
thin section an inch square may be the key to a particular part of the 
problem, but the making of orientation diagrams alone does not get the 
answer; it is their interpretation and their correlation with all other 
petrofabric data of the area under study that complete the picture. 

The limited definition accorded the term "petrofabrics" by some 
workers may have been responsible for this. The term has been defined in 
various ways, and numerous students of the subject give it somewhat differ- 
ent connotations. One of the most complete definitions is to be found in 
Ingerson's short paper "Why Petrofabrics?" ^^ 

But the word as it should be understood in petrofabrics is much more 
comprehensive. In this sense, it is analogous to the fabric of a building, that 
is, its entire make-up from the structure of the individual bricks and the 
mortar that holds them together, to the steel framework that binds the whole 
into a unit. In other words, "fabric" includes all of the spatial relation of a 
rock from the space-lattice of the individual mineral grains through cleavage, 
fractures, joints, schistosity, lineation, and fold-axes. 

Therefore, petrofabrics to be complete should be approached as a 
field problem supplemented by laboratory work. 

As defined above, all the details of a structural unit are brought 
together. True enough, to acquire all data may require a tremendous 
amount of tedius labor. But then, is not the price paid for any item 
controlled by the returns? At the present moment, this tool is being em- 
ployed chiefly by those more academically minded, although its applica- 
tion to economic problems is advancing steadily. 

The metamorphic and igneous rocks have received the greatest atten- 
tion, because the features and principles of petrofabrics are best developed 
in them. Many of these principles probably hold true for the less-deformed 
sedimentary rocks but are not so pronounced and thus have received less 
attention. 

The purpose of this discussion is not to present the techniques and 
methods of petrofabrics in detail but to summarize the rudiments with 
references to literature on various phases. 

" Ingerson, Earl, Why Petrofabrics? : Carnegie Inst. Washington, Geophysics Lab., Paper 1081, 1944. 



158 Subsurface Geologic Methods 

Field Work and Its Presentation 

Some of the more important papers on petrofabric methods and pre- 
senting information on petrofabrics gathered in the field, are the follow- 
ing: "The Application of Recent Structural Methods in the Interpretation 
of the Crystalline Rocks of Maryland," by Cloos,^^ a volume that contains 
not only Cloos' paper on methods but also a number of supporting papers 
by his students; "Lineation," by Cloos,^^ dealing exclusively with linea- 
tion, its formation, interpretation, and mapping; "Oolite Deformation in 
the South Mountain Fold, Maryland," also by Cloos,^^ which is a detailed 
account of the use of deformed ooids in interpreting the structure of an 
area; and "Structural Petrology of Deformed Rocks," by Fairbairn,^^ 
which is a detailed account of petrofabrics, its interpretation, and presenta- 
tion, chapters 1 through 7 dealing with the theoretical aspects of the sub- 
ject. 

The presentation of petrofabric features is accomplished through the 
use of symbols. The United States Geological Survey has recently pub- 
lished a "New List of Map Symbols" that may be obtained free of charge 
from the Geological Map Editor, United States Geological Survey, Wash- 
ington 25, D. C. These symbols were submitted by a committee composed 
of Ernst Cloos, L. B. Pusey, W. W. Rubey, and E. N. Goddard, Chairman. 
Reproduced on pages 160 to 163 are the symbols from this list that are 
most frequently used in mapping petrofabric data. 

Plastic flow in crystalline material takes place by intergranular lattice 
displacement and rotation and migration of materials without destroying 
the cohesion between the grains of the rock subjected to deformation. 
Rocks thus deformed are called "tectonites," whereas all others are classed 
as "nontectonites." 

In order to carry out the correlation between the field data and micro- 
scopic data, the structures of the rock are referred to three coordinates 
or axes a, b, and c, each normal to the other. Figure 60 illustrates a plung- 
ing anticline with its elements tied to these axes. From this diagram it is 
seen that a is the direction of movement or transport, b is the fold axis, 
and c is the vertical component. The a-c plane is normal to the b direc- 
tion. 

While field mapping is in progress, oriented specimens are collected 
from the area for later microscopic study. These are so marked (fig. 61) 
that, once in the laboratory, they are reoriented to their proper position 
in space; thus, the microdata may be correlated with the a, b, and c 
structure axes. 

Figures 62 to 67 show tight folds in bedded, schistose quartzite of the 

^- Cloos, Ernst, The Application of Recent Structural Methods in the Interpretation of the Crystalline 
Rocks of Maryland: Maryland Geol. Survey, vol. 13, 295 pp., 1937. 

°" Cloos, Ernst, Lineation: Geol. Soc. America Mem. 18, 1946. 

^* Cloos, Ernst, Oolite Deformation in the South Mountain Fold, Maryland: Geol. Soc. America Bull., 
vol. 58, pp. 843-918, 1947. 

^ Fairbaim, H. W., Structural Petrology of Deformed Rocks, Cambridge, Mass., Addison-Wesl^y Press, 
Inc., 1912. 



Subsurface Laboratory Methods 



159 



a-c plane 
Angle of plunge 



c vertical 



b - fold axis 




Trace of bedding 
on cleavage plane 



a-c cleavage or 
cross joints 



^ Axial plane cleavage trace 
on bedding plane 



Figure 60. Illustrating a plunging anticline with its elements tied to axes a, b, and c. 




Figure 61. Oriented hand specimen labeled to show its geographic location and 
space orientatior. The top of the specimen, coordinates, and number are 
marked on adhesive tape. Some workers prefer to scratch this information di- 
rectly on the rock. Oriented thin sections are cut from the specimen in the 
laboratory and marked as shown. The section illustrated is cut parallel to the 
a-c plane and normal to the b axis. 



160 



Subsurface Geologic Methods 



FAULTS 



Fault, showing dip 
(Dashed where approximately located) 



Barbs on dip symbol 
may be omitted if pre- 
ferred. 



Vertical fault 



Concealed fault 
V 

Doubtful or probable fault 
(Dotted where concealed) 



Question mark indicates 
uncertainty as to 
existence of fault 



Fault, showing bearing 
and plunge of grooves, striations, 
or slickensides 



Plunge measured in 
vertical plane. 



High angle fault 
(U, upthrown side; D, downthrown side) 



Normal or reverse 
fault. 



Fault, showing relative 
movement 



Fault, showing bearing 
and plunge of relative 
movement of downthrown block 



Normal fault is shown.) 
Reverse fault would 
appear thus: 



Thrust or low angle reverse fault 
(T, upper plate) 



Normal fault 
(Hachxires on downthrown side) 



Thrust or reverse fault 
(Saw-teeth on side of upper plate) 

., # 

Fault zone or shear zone, 
showing dip 

<•« ;;•. 

Fault breccia 



For use on special 
tectonic maps only. 



Suitable also as an 
overprint for 
mylonitized zones and 
broad areas of fault 
breccia. 



Subsurface Laboratory Methods 



161 



FOLDS 
(May be shown in color where 
structure is unusually 
complex) 



Anticline 

(Showing trace of axial plane and 

bearing and plunge of axis. 

Dashed where approximately located) 



If crest line of the 
fold is mapped rather 
than the trace of the 
axial plane, the wording 
should be "showing crest 
line." 



Solid, dashed and 
dotted as on 
anticline 



^ ■■ 

Concealed anticline 

Doubtful or probable anticline 
(Dotted where concealed) 



7-V- 



Syncline 

(Showing trace of axial plane and 

bearing and plunge of axis) 



If position of trough is 
mapped rather than trace 
of axial plauie, the word- 
ing should be "showing 
position of trough." 



Overturned anticline 

(Showing trace of axial plane, direction of dip 

of limbs, and bearing and plunge of a}Ss) 



Overturned syncline 
(Showing trace of axial plane 
and direction of dip of limbs) 



Plunge of minor anticline 

Plunge of minor syncline 

Plunge of fold axes 



Plunge measured in 
vertical plane 



To be used where beds 
are too tightly folded 
to show individual folds 
separately. 



Horizontal fold axes 



162 



Subsurface Geologic Methods 



BEDDING 



Strike and dip of beds 



"trike and dip of overturned beds 



Strike of vertical beds 



It is suggested that this 
symbol be used only where 
the beds are known to be 
right-side up. If it is 
not known which side is 
up, the following symbol 
is suggested: ^-^ 



The position of the 90 
can be used to indicate 
the up side of the beds. 
If so, this should be 
stated in the Explanation. 



Horizontal beds 

scr^ soy 

Generalized strike and dip 

of crumpled, plicated, 
crenulated, or undulating beds 



Strike and dip of beds 
and plunge of slickensideb 



FOLIATION AND CLEAVAGE 



Strike and dip of foliation 



Strike of vertical foliation 
Horizontal foliation 



Strike and dip of cleavage 



To be used for either 
primary or secondary 
foliation. For 
distinguishing between 
various types of planar 
structures, the following 
additional symbols are 
suggested: 

///// 



The type of cleavage 
mapped should be 
specified in the 
Explanation 



Strike of vertical cleavage 



Horizontal cleavage 



Subsurface Laboratory Methods 



163 



LINEATIONS 
(Includes flow lines, alinement 
of minerals, inclusions, streakings, etc.) 



These can also 
be used for 
special type's of 
lineation such as 
intersection of 
planes, wrinklings, 
etc., but such uses 
should be so stated 
in the Explanation. 



Bearing and plunge of lineation 
■-/to 

30* 

Strike and dip of foliation 
and plunge of lineation 



Vertical lineation 

Horizontal lineation 

JOINTS 



Point of observation 
is at base of arrow. 

Plunge measured in the 
vertical plane. If the 
lineation is measured in 
the plane of the foliation 
it is suggested that the 
term rake be used and 
that the symbol be shown 
thus: ^ 



It is recommended that 
the term pitch be aban- 
doned as it has been so 
widely used in both senses 
and appears on many pub- 
lished maps indicating 
the vertical angle. 



Strike and dip of joints 

Strike of vertical joints 

-f 

Horizontal joints 



* On the previous list of survey map symbols, the term "pitch" was applied to 
lineations measured in the vertical plane. However, many comments have been 
received urging that the term "plunge" be used instead, as "plunge" was originally 
defined by Lindgren in this sense and is so defined in the text books of Lindgren 
and Billings. It was also pointed out that "plunge" has rarely been used in any other 
sense, and that an increasing number of geologists are adopting Lindgren's definition. 
For these reasons, the Map Symbol Committee decided to adopt the term "plunge" 
for the angle measured in the vertical plane. This is the measurement that is usually 
recorded on geologic maps, but for special structural problems, some geologists prefer 
to record the angle measured in the plane of the foliation, fault or vein, or in the axial 
plane of the fold. Lindgren and Billings used "pitch" for this angle. However, "pitch" 
has been so widely used in both senses and has appeared on so many published maps 
indicating the vertical angle, that its continued use is likely to lead to further con- 
fusion. Therefore, after wide discussion with structural geologists, the committee 
decided to abandon the term "pitch" and to suggest the use of the term "rake" for 
the angle measured in the plane of the structure. This term has been occasionally 
used to describe the inclination of ore bodies, but it has never been clearly defined. 



164 



Subsurface Geologic Methods 



Bell series in the Avery district of Shoshone County, Idaho. The elements 
of these folds can be readily related to the three above-mentioned axes. 
The folds shown are comparatively small and simple; the features 
such as lineation, jointing, and schistosity are pronounced. Careful de- 
tailed mapping may demonstrate these on large, complicated structures. 
The use of such components correlated with microfabrics permits the 
reconstruction of complexly deformed units. 



Laboratory Techniques and Presentation of Data 

Once the macrocomponents are determined, the microscope is re- 
sorted to for further aid. The following publications present the labora- 




FiGURE 62. Tight fold in white schistose quartzite. Schistosity is produced by 
parallel flakes of muscovite. The lineation is parallel to b and is shown in 
figure 63. 



Subsurface Laboratory Methods . 165 

tory techniques of determining microfabric and the methods of its pres- 
entation: "Laboratory Technique of Petrofabric Analysis," by Ingerson,^^ 
which begins with the study of the hand specimen after it has been oriented 
in the field and carries the reader through the steps of microscopic analysis 
of the specimen and recording and presentation of the data; "Structural 
Petrology of Deformed Rocks," by Fairbairn,^^ chapters 8, 9, and 10, giv- 
ing much the same information as Ingerson's paper but with a somewhat 
different presentation; and "Federow Method (Universal-Stage) of In- 
dicatrix Orientation," by Haff,^^ a paper presenting the universal-stage 
and its operation. 

In the laboratory the oriented hand specimens collected as described 
above are studied megascopically or with the binocular or by both methods 
to determine the selection of coordinates. The most prominent structure 
plane such as schistosity is taken as the a-6 plane. The direction for the 
thin sections are determined by the choice of the three reference axes. 
Generally three sections are cut normal to a, b, and c (fig. 61). If there 
is doubt as to the selection of the axes, random thin sections may be cut 
and studied in order to discover any preferred orientation. 

Quartz, calcite, and the micas are the minerals commonly used for 
the construction of orientation diagrams. In quartz the attitude of the 
optic axis is determined; in calcite, the optic axis or the poles (the normal 
to) of twin or glide planes are mapped; in the micas the poles of the 
cleavage (010-plane) planes are plotted. The measurements of these 
features are carried out on the universal stage mounted on a petrographic 
microscope; the results are plotted on a Schmidt net (fig. 68). This net is 
the equal-area, azimuth projection of the lower half of a sphere. 

If quartz orientations are being studied, the direction of the c-axis 
is determined on the universal-stage. This axis is brought into coincidence 
with the microscope axis; its original attitude may then be read from 
the graduated circles of the U-stage. These values are a bearing and an 
angle of inclination, which are then plotted on the net. Both are repre- 
sented by a point on the diagram that in a three-dimensional solid would 
be the point at which the c axis pierces the lower hemisphere. If mica 
books are to be oriented, however, the cleavage planes are brought into 
line with the plane containing the microscope axis, and the attitude of 
the poles (the normals to) of the cleavage planes is plotted in much the 
same manner as the quartz c axes. 



^° Ingerson, ilarl, Laboratory Technique of Petrofabric Analysis (pt. 2 of Structural Petrology by 
Knopf, E. B., and Ingerson, Earl) : Geol. Soc. America Mem. 6, pp. 209-262, 1938. (A separate of this 
is Carnegie Inst. Washington, Geophysics Lab., Paper 959.) 

" Fairbaim, H. W., Structural Petrology of Deformed Rocks, pp. 106-131, Cambridge, Mass., Addison- 
Wesley Press, Inc., 1942. 

^' Haff, J. C, Federow Method (Universal-Stage) of Indicatrix Orientation: Colorado School of 
Mines Quart., vol. 37, no. 3, pp. 3-28, July 1942. 



166 



Subsurface Geologic Methods 



Normally two or three hundred grains are oriented and plotted for 
each thin section. Upon completion of plotting, the concentration of 
points is contoured. That is, the concentrations according to percentages 
are separated by lines having values such as 1, 2, 3, and 4 percent. The 







Figure 63. Detail of crest of fold in figure 62. The scale is parallel to the b axis. 
The lineation is caused by the intersection of the schistosity and a bedding 
plane. The joints normal to the b axis are a-c joints. 



area for each percentage is given a pattern, and the greatest density is 
generally shown in solid black. This procedure completes the orientation 
diagram except for marking the reference plane, which must be clearly 
shown. 

Petrofabric diagrams are classed as "elemental" when a single mineral 
is studied in one thin section, and as "collective" when the plotted points 
from a number of elemental diagrams are combined into a single diagram. 



Subsurface Laboratory Methods 



167 



In general, statistical diagrams of quartz show (fig. 69) either iso- 
lated maxima or a distribution of axes in bands that may or may not 
contain maxima. Fairbairn,^^ has an illustration in his book that shows 
the types of quartz diagrams possible from thin sections cut normal to the 
reference axes a, b, and c. 




1 ..^^b^.iS^ 



Figure 64. Looking south along b axis of a tight, slightly overturned fold in bedded, 

schistose quartzite. 



The micas (biotite, muscovite) ordinarily show a complete or partial 
girdle (fig. 70) around b parallel to the a~c plane with a maximum at c. 

Figure 71 is a schematic drawing of the fold shown in figure 64. 
This fold is in a schistose, white quartzite, in which parallel muscovite 
flakes produce the schistosity. The simplified petrofabric diagrams super- 



^' Fairbaim, H. W., Structural Petrology of Deformed Rocks, p. 8, Cambridge, Mass., Addison-Wesley 
Press, 1942. 



168 



Subsurface Geologic Methods 



imposed normal to the a, b, and c fold axes are the types one may expect 
from the statistical study of the muscovite books. 







Figure 65. Lineation on west or upper limb of fold shown in figure 64. The hori- 
zontal lineation is due to intersection of bedding (dipping 45° out of the picture) 
and schistosity (dipping 50° out of picture). The lineation almost normal to it is 
caused by quartz filling tension joints normal to the b axis and opened by 
stretching along that axis. The fold shown in figure 64 was discovered by careful 
mapping of the lineation and schistosity. 



Possible Applications 

Detailed structural studies to be complete should make thorough 
use of petrofabrics. The individual techniques of petrofabrics, however, 
may be used separately on special problems. 

Ingerson ^^ gives a list of possible applications of petrofabrics. There 

*" Ingerson, Earl, Why Petrofabrics? : Am. Geophys. Union Trans., vol. 25, pp. 636-652, 1944. 



Subsurface Laboratory Methods 



169 



are additional uses, and with the advance of the science still others will be 
found. 

From magnetically oriented drill cores (see "Magnetic-Core Orienta- 



■^■*i 





i '■' #'- 



Figure 66. Looking along b axis of an isoclinal fold in the same white quartzite as 
shown in figures 62-65. The schistosity and the bedding planes intersect at a 
low angle and produce prominent lineation as shown in detail in figure 67. 



tion") oriented thin sections can be obtained. The statistical analysis of 
such thin sections tied in with other known petrofabric data will help 
work out subsurface structures. 

Hohlt,^^ in a recent paper on limestone porosity, made statistical 
studies of carbonate rocks with the idea of correlating mineral orientation 
and porosity. He concluded that orientation is related to dolomitization. 

^' Hohlt, R. B., The Nature and Origin of Limestone Porosity: Colorado School of Mines Quart., 
vol. 43, no. 4, Oct. 1948. 



170 



Subsurface Geologic Methods 



The study of sedimentary rocks by petrofabric methods has been 
comparatively neglected. Ingerson ^~ indicates that valuable information 
on current direction may be gained from the fabric study of sediments. 





Figure 67. Pronounced lineation parallel to b axis of fold. The angle this lineation 
makes with the horizontal gives the plunge of the fold. The joints normal to the 
lineation are a-c joints. 



Wayland ^^ has shown that tlie tendency of the long axis of quartz 
grains is to be parallel to the c axis and that the c axis lies nearly parallel 
to the bedding plane in sandstones. 

Size and shape of fragmental materials in clastic sedimentary rocks 



^- Ingerson, Earl, Fabric Criteria for Distinguishing Pseudo-Ripple Marks from Ripple Marks: Geol. 
Soc. America Bull., vol. 51, pp. 557-570, 1940. 

^ Wayland, R. G., Optical Orientation in Elongate Clastic Quartz: Amer. Jour. Sci., vol. 237, pp. 
99-109, 1939. 




Figure 68. Reproduction of a Schmidt net of 20-centimeter diameter for use in 
petrofabric work. (After Cloos.) 




Figure 69. 470 quartz axes showing Figure 70. 134 cleavage poles of bio- 
maxima about b. Contours 4-2-1 per- tite forming a girdle parallel to a-c 
cent. (Modified from Fairbairn.) with maximum about c. Contours 8-6- 

4-2-1 percent. (After Fairbairn.) 



172 



Subsurface Geologic Methods 



need further study. Papers by Ingerson and Ramisch,^^ Anderson,^^ and 
Ingerson and Tuttle ^^ point to the origin of quartz grain shapes. 

MICRO (PETROGRAPHIC) ANALYSIS 
WARREN R. WAGNER and JOHN W. GABLEMAN 

Microscopic analysis of rocks is such a well-established field of study 
that this paper is not intended to be an exhaustive presentation of the 
methods developed to the present time. The writers wish only to call to 




Figure 71. Schematic drawing of the fold in figure 64, showing in simplified form 
the types of statistical diagrams to be expected from mica flakes in a tight fold. 



the attention of students and workers some of the applications of the micro- 
scope to rock examinations. 

Many micro-techniques are time-consuming and expensive; therefore, 
the first duty of the investigator is to decide upon the method that will 
produce the desired information with the minimum cost. 

In general, two types of microscopes are availiable: (1) the petro- 

" Ingerson, Earl, and Ramisch, J. L., Origin of Shapes of Quartz Sand Grains: Am. Mineralogist, vol. 
27, pp. 595-606, 1942. 

^ Anderson, J. L., Deformation Pliines and Crystallographic Directions in Quartz: Geol. Soc America 
Bull., vol. 56, pp. 409-430, 1945. 

^* Ingerson, Earl, and Tuttle, 0. F., Relations of Lamellae and Crystallography of Quartz and Fabric 
Direction in. Some Deformed Rocks: Am. Ceophya. Union Trans., vol. 26, pt. 1, pp. 95-105, 1915. 



Subsurface Laboratory Methods 



173 



graphic miscroscope and (2) the binocular microscope. The former 
makes use of polarized light for the identification of minerals and the 
detailed study of thin sections, whereas the latter uses ordinary trans- 
mitted or incident light and is normally used for observing the larger 
features such as lithology, texture, and structure. 

Some rivalry appears to exist among workers as to the superior quali- 





FiGURE 72. Basic equipment required for preparing a rapid 
heavy-mineral concentrate of a rock. 



ties of the two types of microscopes. Each type has its field of usefulness. 
Numerous problems are solved most economically with the aid of both 
kinds of microscopes. Any well-equipped microscopic laboratory should 
contain both petrographic and binocular microscopes, and should have 
trained technicians trained to operate them. 

On pages 119-120, Rittenhouse discusses some of the uses of the 
binocular microscope on sedimentary rocks. His article deals primarily 
with detrital mineralogy, whereas this paper is chiefly concerned with 
thin-section investigation, although a rapid method of heavy-mineral sepa- 
ration is presented. 



174 Subsurface Geologic Methods 

Rapid Method of Heavy-Mineral Separation ^^ 

One of the major drawbacks to heavy-mineral studies is the time 
required in making clean separations. A competent worker with the proper 
setup can make a complete heavy-mineral separation, including the per- 
manent micro-mount, in a lapsed time of 10 minutes by following the steps 
outlined and illustrated below. 

Necessary Equipment 

Figure 72 shows the equipment required in the procedure for making 
a single separation. The articles shown are bromoform, a standard 3-inch 
evaporating dish, stainless-steel teaspoon, filter paper, product to be con- 
centrated, wash bottle with alcohol, adjustable wooden rack, and glass 
funnels and beakers for filtering. The setup may be varied to fit the size 
of the problem. If a number of separations are to be carried on, the 
process may be speeded up by having the workbench arranged for multi- 
ple equipment. 

Procedure 

Figure 73, steps a, h, c, d, e, and /, illustrate the stages of making a 
separation. Step a: The prepared and weighed sample is placed in the 
evaporating dish with sufficient bromoform (diluted to desired density) 
to float the light fraction freely. The material is then stirred thoroughly 
with the teaspoon in order that the "heavies" may sink. Step h: After 
the sample is stirred, it is allowed to settle for one-half to one minute, 
and the larger part of the floating light fraction is spooned off into the 
filter. Step c: The remaining "lights" are then carefully poured off in 
successive order with a few seconds of gentle circular motion (panning) 
between each pouring. Step d: The filter containing the bromoform and 
"lights" is then held above the evaporating dish to allow the bromoform 
that is filtering out to wash down the sides of the dish. This step is repeated 
until the separation is complete and the spout or pouring side of the dish is 
free from the light fraction. Step e: The final concentrate is washed from 
the dish into the second filter with alcohol from the wash bottle. The 
"'heavies" are then washed clean of bromoform, filtered, and dried on a 
hot plate. Step f: The final, dried, heavy fraction is then divided and 
mounted as desired. 

If proper caution is exercised throughout the procedure, a clean sepa- 
ration is obtained. Bromoform is expensive and care should be taken to 
avoid wastage. 

Preparation of Thin Sections 

A thin section is a slice of rock or mineral 0.03 millimeter thick 
mounted on a glass slide for examination under the petrographic micro- 
scope. The nonopaque, rock-forming minerals have a high degree of trans- 

*' Adapted from procedure teughl the senior author by Dr. J. L. Anderson, Department of Geology, 
Johns Hopkins University. 







step d 




v--t 



. Step e Step f 

Figure 73. Procedure followed in making a rapid heavy-mineral separation. 



176 Subsurface Geologic Methods 

parency in thin slices, and their different reactions to transmitted, polarized 
light constitutes the basis of optical mineralogy. Because the optical 
properties of the minerals differ with the thickness of the section, 0.03 
millimeter has been selected as an arbitrary standard thickness. 

Thin sections may be made of well-consolidated rocks, of friable or 
less well-consolidated rocks, of individual minerals, of fragments of min- 
erals or rock, or of a heavy-mineral concentration. The friable or poorly 
consolidated material is impregnated with a bonding cement before sec- 
tioning, whereas the fragmental material or heavy-mineral concentrate 
requires a special technique that will be described later. 

The equipment necessary for preparing thin sections in the labora- 
tory include a diamond power saw, power-lap wheels of cast iron, glass 
plates, a hot plate, and a microscope. Materials needed are mounting 
cement, abrasives, glass mounting slides, and cover glasses. 

The number of laps and grades of grinding compound used depends 
upon the technique to be followed. At least two grinding laps are re- 
quired, one each for course and fine abrasives. The technique briefly out- 
lined below is used at the Colorado School of Mines; it may be varied to 
suit the needs of individual specimens. 

Sawing and Grinding 

From the rock specimen to be studied, saw a piece 3 to 5 millimeters 
thick with the two parallel, flat faces. Trim the edges until the final slice 
measures approximately 30 x 22 x 3 mm. Now choose the smoothest side 
and, with 80- to lOO-mesh abrasive on the course lap wheel, grind this 
face until all traces of the saw marks disappear. Wash the slice thor- 
oughly, transfer to the 320-mesh abrasive on the fine lap wheel, and polish 
the ground surface. The polished surface is to be mounted next to the 
glass slide and, therefore, must be perfectly flat. A final polish with 600- 
mesh abrasive is often required. The worker must learn from experience 
when this final polish is necessary. 

When the grinding is completed, wash the slice thoroughly to remove 
all abrasive and foreign material. A tooth brush is helpful for this. The 
shaped and polished piece now has one flat, smooth surface ready for 
mounting.''^ 

Mounting the Slice 

Place the rock slice on a hot plate w^ith the polished side up and ex- 
clude all moisture. When drying is complete, place a standard petro- 
graphic glass slide (45 x 25 mm.) beside the specimen and allow the tem- 
perature of both to become the same. (If a controlled-temperature hot 
plate is available, keep it at about 300° F.) Cover the upper surface of 

^ The abrasives are kept in kitchen-size salt shakers. Abrasive is applied to the wet lap wheel as 
needed. Some workers mix it with water in a bottle provided with a glass tube through the cork in order 
that the contents of the bottle may be shaken on to the lap as required. Experience is necessary before 
the operator can obtain the right mixture of water and abrasive that will produce the most efficient 
cutting. 



Subsurface Laboratory Methods 177 

both the rock slice and the glass slide with an even, thin layer of raw 
Canada balsam and cook. If the rock slice is porous, apply an excess of 
balsam. The hot plate should be kept level so that the balsam may flow 
evenly in all directions. 

The success of mounting the specimen lies in cooking the balsam. If 
it is insuflficiently done, the rock slice will not adhere to the glass slide; 
if it is overdone, the balsam will be too brittle and will break away when 
ground. The object is to cook the balsam to the point that, when it is 
cooled, one may barely dent it with the thumbnail. While cooking is in 
progress, the cement is continually tested by taking a small amount on 
the end of a toothpick and biting it between the front teeth. When balsam 
is properly cooked, it will stick to the teeth, barely begin to pull, and then 
abruptly break. In other words, it becomes "tacky." ^^ 

When the cement is properly cooked, place the rock slice on the glass 
slide with the two balsam-covered sides in contact. Remove the mount 
from the hot plate to an asbestos pad. With the eraser end of a pencil, 
work the rock slice around, meanwhile applying considerable pressure 
in order that the air bubbles and excess balsam will be squeezed from 
between the slice and the slide. Center the slice and allow the cement to 
set while pressure is applied. As the mount cools, test the balsam for 
hardness with the thumbnail. Cooling should be allowed to proceed nor- 
mally, as sudden changes in temperature tend to pull the rock slice from 
the glass. The mount is now ready to be ground to the desired thinness. 

Grinding to 0.03 Millimeter 

The final grinding to 0.03 millimeter is carried out in three stages. 
The mount is ground on the coarse lap with 80- to 100-mesh abrasive 
until it is about 0.10 millimeter thick. It is then cleansed carefully and 
transferred to the fine lap wheel with 320-mesh abrasive. During this 
stage, with proper caution, a well-cemented slide may be ground almost to 
the desired thinness. The writers ordinarily carry the grinding with 320- 
mesh abrasive to a point where such colored minerals as hornblende or 
biotite are fairly transparent when held before a light source. Further 
grinding by mechanical means becomes hazardous; therefore, the last 
stage of grinding is done on a glass plate with 600-mesh abrasive in water. 
The section is moved with a gentle, circular motion on the plate. If the 
slide has become wedge-shaped, more pressure is applied on the thicker 
portion to bring the entire rock slice to the same thickness. As the slide 
is now very thin, constant checking for thickness under the microscope is 
necessary. Quartz is the mineral commonly used as an index, and the sec- 
tion has reached 0.03 millimeter when quartz shows a faint, straw-yellow, 
interference color. 



balBam 



Other cements are available, but the writers have had the most consistent results with raw Canada 



178 Subsurface Geologic Methods 

Mounting the Cover Glass 

The concluding operation in preparing a thin section is to mount the 
cover glass. The simplest method of mounting the cover glass is to use 
cooked balsam dissolved in xylene. To cement the cover glass, place a 
thin, even layer of prepared balsam on the surface of the section. Put the 
cover glass in place and press down sufficiently with the eraser end of a 
pencil to remove all air bubbles and excess balsam. After removing the 
squeezed-out balsam, the slide is complete. Several days may be required 
for the xylene to evaporate and for the balsam to set; but with proper pre- 
cautions, the section can be used immediately. Until the balsam is dry, the 
slide should be stored in a flat position. 

Cover glasses are obtainable in various sizes and thicknesses. For 
most purposes the No. 1 thickness is preferable. 

Thin Sections of Fragments or Heavy Concentrates 

The investigator may find that fragmental materials and heavy-min- 
eral concentrates contain minerals which defy identification by physical 
or optical means in their present state. If the material is such that a thin 
section may aid identification, it is not difficult to make one from even 
rather finely divided substances. The process and materials used are much 
the same as those used in making an ordinary thin section, except that no 
sawing is required and the mounting procedure is repeated for the second 
time. 

To make a thin section of a heavy-mineral concentrate, place a petro- 
graphic, glass, mounting slide on the hot plate. Put raw Canada balsam 
on it and cook. Immediately before the balsam is sufficiently cooked, sprin- 
kle an excess amount of the heavy concentrate into the hot balsam and 
permit the grains to settle. An excess is required as some material will be 
lost in a later transfer. For this step, the cooking of the balsam is not 
critical as this material is merely a holding mount. Remove the slide from 
the hot plate and allow to cool. 

The next step is grinding a flat, permanent-mounting surface on the 
mineral grains with a 150- to 200-mesh abrasive. The amount of grinding 
necessary depends upon the grain size; if the grains are of uniform size, 
one should grind about halfway through them. The technician must learn 
the required amount to be ground by experimenting. An inspection under 
a low-power microscope may prove useful in determining whether or not 
the surface on the grains is suitable. If not, a transfer to 320- or even 
600-mesh abrasive may be essential. (The entire grinding procedure may 
be carried out on glass plates with the desired size abrasive.) 

Assuming that the ground surface is now flat and polished for mount- 
ing, the slide is cleaned of all foreign material. Now moisten a cloth with 
xylene and slowly dissolve the excessive cement from between the grains; 
leave only sufficient balsam to hold the grains in place. 

The next step is remounting the grains on a new petrographic slide 



Subsurface Laboratory Methods 179 

with the flat surface on the glass. This operation requires some practice, 
as it must be accomplished quickly and at a given time. To accomplish 
the remounting, a glass slide covered with an even, thin layer of balsam 
is placed on the hot plate and the balsam is cooked within an "instant" of 
the required consistency. The first mount is then laid upon the new slide 
with the flat surface of the grains in contact with the newly cooked balsam. 
It is allowed to remain there until the undissolved balsam melts. (The time 
required here is short; the new balsam should be correctly cooked upon 
completion of the operation. One must remove rapidly.) The slides are 
now removed to an asbestos pad; the upper one (the temporary mount) 
is pressed down, moved with a circular motion, and gently slid in the direc- 
tion of the narrowest dimension off" the lower slide. A small quantity 
of mineral grains may adhere to the first mount, but their loss is not seri- 
ous as an excess of grains were originally used. For the final step, the 
slide is allowed to cool. Then it is ground to the required 0.03 millimeter 
in thickness, and the cover glass is mounted as recommended for an ordi- 
nary thin section. 

Manual Preparation 

In the foregoing procedure for thin-section preparation, a laboratory 
with power equipment was assumed. Thin sections are often needed when 
this equipment is not available. Excellent sections may be prepared manu- 
ally from rock chips or even small well cuttings by carrying out the grind- 
ing operations on glass plates. The manual procedure is often to be pre- 
ferred with friable material, very small chips, or soft rocks such as lime- 
stones and shales. For base-camp use, a small, compact grinding outfit 
can be assembled with which a careful worker can make thin sections of 
any desired rock. 

Thin-Section Study 

A complete petrographic analysis requires the use of every available 
tool. The use of thin sections is merely one phase in the breakdown of 
a rock into its component parts. It is, however, an important phase, and 
usually the choice of the final method of study to be employed in the study 
of a given rock is determined from thin sections or a combination of thin 
section, polished surface, and hand specimen. The finer-grained sediments 
such as shales, siltstones, and mudstones yield comparatively little infor- 
mation to the investigator in thin section, whereas, the medium-grained 
clastic rocks offer a fertile field for micro-study. The flow sheet (fig. 74) 
shows the organization and methods that may be used in a modern petro- 
' graphic laboratory. 

Some of the more important data and information that may be ac- 
quired from thin-section studies of sedimentary rocks are summarized in 
the paragraphs below. 




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Subsurface Laboratory Methods 181 

Rock Classification '^^ 

One of the first aims of the petrographer is to classify the rock as to 
type, i.e., sandstone, calcareous sandstone, argillaceous sandstone, etc. The 
typing or naming of the rock in itself yields much information about its 
internal makeup; and, incidentally, the thin section will usually reveal 
sufficient information on the clastic materials of any size for classification 
purposes. 

Cementation 

Waldschmidt,'^^ in his paper on cementing materials of sandstones in 
the Rocky Mountain region, has worked out a sequence of deposition for the 
various binding materials. He also shows their relation to porosity and 
permeability in sandstones having various combinations of these cement- 
ing minerals. His order of deposition of cementing minerals is as follows : 



One 


Two 


Three 


Four 


cementing 


cementing 


cementing 


cementing 


mineral 


minerals 


minerals 


minerals 


Quartz 


1st Quartz 


1st Quartz 


1st Quartz 




2nd Calcite 


2nd Dolomite 


2nd Dolomite 






3rd Anhydrite 


3rd Calcite 






or 


4th Anhydrite 






1st Quartz 








2nd Dolomite 








3rd Anhydrite 





Waldschmidt's conclusions are based on the study of 111 sections. 
His paper is an excellent illustration of the use of thin sections in this 
type of study. 

Porosity and Permeability "^^ "^^ 

Many of the principles governing porosity and permeability go be- 
yond the realm of thin-section study; yet, much of the information as to 
why and how certain rocks have as much or little porosity and permea- 
bility may be learned from the study of thin sections. Grain size and ar- 
rangement or packing, authigenic mineralization, alteration, interstitial ma- 
terials, and cementation all exert considerable influence on the amount of 
pore space available in a rock. 

A single thin section presents a two-dimensional view; therefore, for 

'" Pettijohn, F. J., Sedimentary Rocks, New York, Harper and Brothers, 1949. 

'^ Waldschmidt, W. A., Cementing Materials in Sandstones and Their Probable Influence on Migration 
and Accumulation of Oil and Gas: Am. Assoc. Petroleum Geologists Bull., vol. 25, no. 10, pp. 1839-1879, 
1941. 

" Graton, L. C, and Fraser, H. J., Systematic Packing of Spheres with Particular Relation to Po- 
rosity and Permeability: Jour. Geology, vol. 43, pt. 1, pp. 785-909, 1935. 

" Fraser, H. J., Experimental Study of the Porosity and Permeability of Clastic Sediments : Jour. 
Geology, vol. 43, pt. 1, pp. 910-1010, 1935. 



182 



Subsurface Geologic Methods 



complete studies three sections each normal to the other should be cut 
from a specimen. It is also an excellent idea to polish the surface of the 
specimen from which each section was originally sawed and to study 
these under the microscope. If the data obtained from these studies are 





JKe-'aAJ 






Figure 75. Photomicrographs of thin sections from four sandstones of Morrison 
formation near Golden, Colorado, illustrating morphological differences. Note 
chalcedonic cementation in upper right. 



recorded on a block diagram, any directional variation in space relation- 
ships shows up more readily. 



Subsurface Laboratory Methods 183 

Thin-Section Correlation 

Consolidated sedimentary rocks are often sufficiently individualistic 
enough that a careful megascopic inspection is adequate for correlation 
purposes. On the other hand, in a thick, monotonous series of shales, of 
limestones, of sandstones, or of combinations of these, megascopic identi- 
fication and correlation are frequently uncertain or impossible. Identifica- 
tion and differentiation of a specific horizon may be accomplished by a 
microscopic study of thin sections. Possible microscopic evidences that 
may be used include textures, structures, cementing minerals, detrital 
material, degree of crystallization, amount of recrystallization, finer or- 
ganic forms, and other morphological criteria. The photomicrographs of 
figure 75 illustrate a few of the microscopic features of four different sand- 
stones. 

Vug and Opening Studies 

Normal-sized thin sections are limited in area. Ordinarily, in petro- 
graphic work the slides used are 45 x 25 millimeters, and the finished rock 
section rarely covers more than two-thirds of this area. For vug and open- 
ing studies, larger areas are required. The writers have made and used to 
advantage thin sections mounted not only on a 2 x 2-in. kodachrome, slide- 
cover glasses, but also on standard 3^ x 4-in. lantern, slide-cover glasses. 
These sections are usually somewhat thicker than 0.03 millimeter, but they 
may be projected in an ordinary slide projector equipped with polaroids. 
If the sections are cut in parallel series, a fair picture of the size and con- 
tinuity of "vugation" is presented. 

Storage of Sections 

Problems of storage of specimens and sections for future reference 
continuously plague the research worker. Each man has his own solution. 
One space-saving device is to mount the thin section, heavy-mineral con- 
centrate, and a chip sample from the same specimen on a three-inch bio- 
logical glass slide rather than on the conventional petrographic slide. 

Summary 

Thin sections are not the answer to all sedimentary problems, but they 
disclose the internal view of a rock from which the research man may add 
to his present knowledge of the rock; and at the same time, they aid in 
determining his future mode of attack. 



184 Subsurface Geologic Methods 

SIZE ANALYSIS 
L. W. LeROY 

Size analysis permits comparison of grain-size similarities and 
dissimilarities of sands and dissaggregatable sandstones. With regard to 
results obtained by size analysis, Twenhofel and Tyler ''^^ comment: 

Statistical analyses certainly permit rapid and easy comparison of large 
numbers of sediments and render it simple to point out similarities and differ- 
ences. The best that may be stated is that the significances of the studies 
are not apparent. Statistical studies certainly permit extensive use of mathe- 
matical formulae which are of interest to those mathematically inclined. The 
writers have found these formulae of great interest but not particularly useful 
so far as interpretation of the sediments is concerned. 

According to Pettijohn "^^ the purposes and significance of size anal- 
yses are as follows: 

(1) The improvement of classification and the precision of nomencla- 
ture of clastic sediments, (2) the study of the influence of grain-size dis- 
tribution on porosity and permeability, (3) the study of relations between 
the dynamics of stream flow and the transportation of particulate materials, 

(4) quantitative studies of facies changes and correlation problems, and 

(5) identification of the agent or environment responsible for the origin 
of the sediment. 

In the Lake Maraciabo Basin of western Venezuela, size-analysis data 
locally reflect the contact between the El Milagro (Pleistocene) and Onia 
(Pliocene) formations and between the Onia and La Villa (Miocene) 
(fig. 76). The Lyons (Permian) and Fountain (Pennsylvanian) contact 
east o*f the Front Range of Colorado may be locally differentiated (fig. 
77) . The classification of soils has been based on the grade-size principle. 
The procedure is utilized as a basis for the computation of the most effi- 
cient size of casing perforations in petroleum-production problems and 
for the selection of gravel-packing installations in water wells. 

Fine-grained elastics (particles less than 0.088 mm. in diameter) may 
be graded by decantation or by elutriation methods. Pipette and hydro- 
meter techniques have also been employed for fractionating fine materials. 

The Wentworth size classification given in table 4 (p. 187) has been 
widely adopted for defining grain-size fractions of clastic sediments. 

Preparation of Sample 

A 300-gram sample is disaggregated by carefully crushing the sample 
in a mortar with a rubberized pestle. Grinding should be minimized to 
prevent excessive grain breakage. Checking the aggregate periodically 
by microscope is essential to insure normal and complete disaggregation. 

The material is placed in a nest of U. S. or Tyler sieves. The sieve 
series may be hand-shaken or placed on the "Ro-Tap," a mechanical vibra- 

''* Twenhofel, W. H., and Tyler, S. A., Methods of Study of Sediments, p. 120, New York, McGraw- 
Hill Book Co., Inc., 1941. 

'^ Pettijohn, F. S., Sedimentary Rocks, p. 30, New York, Harper & Brothers, 1949. 



Subsurface Laboratory Methods 



185 



O 

CD 

< 



< 

o 





^VWr-rrV^ 



7lZ]zZ2=t=o=r=r!22=a 



I I I I 







Hvm^ I r^ 




r r I I I I I I 



Figure 76. Histograms demonstrate grade changes across formational boundaries. 
The data shown are based on continuous ten-foot ditch samples from depths of 
about 1,200 feet. Maracaibo Basin, Venezuela. 




Grad* Sizi In MillimtKri 



Subsurface Laboratory Methods 



187 



TABLE 4 
Wentworth Size Scale 



Type of sediment 


Size limit (mm.) 


Sediment 


Rudaceous 

Arenaceous 

Siltaceous 

Argillaceous 


+256 
256-64 
644 

4-2 

2-1 

1-1/2 

1/2-1/4 

1/4-1/8 

1/8-1/16 

1/16-1/256 

-1/256 


Boulder 
Cobble 
Pebble 
Granule 

Very coarse sand 
Coarse sand 
Medium sand 
Fine sand 
Very fine sand 

Silt 

Clay 



TABLE 5 
Comparison of Tyler and U. S. Sieves 



Tyler 


sieves 


U.S. sieves 


WentwoTth 


Mm. 


Mesh 


Mesh 


classification 


3.96 


5 


5 




3.33 


6 


6 


Granule 


2.79 


7 


7 




2.36 


8 


8 




1.98 


9 


10 




1.65 


10 


12 


Very coarse sand 


1.40 


12 


14 




1.17 


14 


16 




0.991 


16 


18 




0.833 


■20 


20 


Coarse sand 


0.701 


24 


25 




0.589 


28 


30 




0.495 


32 


35 




0.417 


35 


40 


Medium sand 


0.351 


42 


45 




0.295 


48 


50 




0.246 


60 


60 




0.208 


65 


70 


Fine sand 


0.175 


80 


80 




0.147 


100 


100 




0.124 


115 


120 




0.104 


150 


140 


Very fine sand 


0.088 


170 


170 




0.074 


200 


200 




0.061 


250 


230 


Silt 



188 



Subsurface Geologic Methods 



tor equipped with an automatic clock for time control. Results are more 
complete if the material first is placed on the "Ro-Tap" and the final sep- 
aration then completed by hand. After the separation stage, the weight 
of material retained on each screen is determined, and the results are 
tabulated as shown in table 6. 



TABLE 6 

Results of Size Analysis Graphically Shown in Figure 80 
Sample No.: P-64 Lithology: Clear quartz sand Formation: Dakota 



Date: Nov. 26, 1948 



Sample weight: 500 grams 



Analyzed by: W. Stuart 







Grams 


Percentage 


Accumulative 


Mesh 


Mm. 


retained 


retained 


percentage 


28 


.589 


0.3 


0.06 


0.06 


32 


.495 


0.2 


0.04 


0.10 


48 


.295 


2.4 


0.48 


0.58 


60 


.246 


9.9 


1.98 


2.56 


100 


.147 


374.0 


74.80 


77.36 


115 


.124 


43.3 


8.67 


86.03 


150 


.104 


38.3 


7.66 


93.69 


170 


.088 


17.5 


3.50 


97.19 


200 


.074 


6.7 


1.34 


98.53 


-200 




7.4 
500.0 


1.47 


100.00 




100.00 





Plotting of Data 

Three methods of graphic representation are followed in illustrating 
size-analysis data, the histogram, the simple-frequency curve, and the 
cumulative-frequency curve. In each, weight percentages of the various 
grades are plotted against dimension with the former represented on the 
vertical axis and the latter on the horizontal axis. 

Histogram Plot 

The histogram method requires cross-section paper, with the largest 
grade size being placed on the left (figs. 78 and 80) . Each grade percent 
is designated by rectangular blocks, the height of which represents the 
percentage by weight of the respective grade. Histograms are useful for 
rapid visual comparison. The shape of the diagram is controlled by the 
number and percentage of grade fractions involved. For comparative work 
a uniform scale should be employed. Another type of histogram involves 
various percentage blocks laid end on end along the horizontal axis with 
each grade block graphically symbolized (fig. 81). 

Simple-Frequency Curve 

The simple-frequency curve is commonly constructed on cross-sec- 
tional paper (fig. 80). The weight percentage is plotted on the vertical 
axis and the grade value on the horizontal. Points are connected by a 



Lyons (E) 



Lyons (C) 





Fountain ( 


B) 








1 






1 













40- 
30- 
20 
\0-\ 



Fountain ( A) 



.833 .417 .246 .175 .147 .124 -.124 
Grode Size in mm. 



Fox Hills (F) 



50- 

40- Corlile (D) 
30- 










20- 








10- 








1 . . 1 1 



.589 .495 .295 .246 .147 .124 .104 .088 .074 -.074 

Figure 78. Typical histograms of various sandstones of Rocky Mountain 
region. Refer to figure 79 for corresponding cumuJative-frequency- 
curve data. 



190 



Subsurface Geologic Methods 



smooth curved line. This plot is a more accurate representation of sedi- 
ment-grade relationships than is shown by the histogram. 

Cumulative-Frequency Curve 

The cumulative-frequency curve is constructed as follows (fig. 79) : 
The weight percentage of the sample retained on the coarsest sieve is 
plotted first, then the percentage by weight retained on the coarsest sieve 
plus that retained on the next finer sieve, etc. Each point represents the 
total weight percentage of material that would be retained if only the 
sieve represented by that particular point were used in the analysis. 

The cumulative-frequency curve is practically independent of the 



'*i- 


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80 « 
75 5 



o o o o o 

Q Q O Q O. 



Diometer In mm. 



Figure 79. Cumulative-frequency curves of several sandstones of Rocky Mountain 
region; note variation of slope and coefficient of sorting. 

grade scale used and is thus a more reliable index of the nature of the 
particle distribution in sediments than a histogram or simple-frequency 
curve. 

Two-, three-, or four-cycle semilogarithmic paper is used in plotting. 
Data from numerous samples may be plotted on the same base to permit 
direct comparisons (fig. 79). 
Computations 

Krumbein "^^ comments that 

Some workers have used cumulative curves in a purely descriptive manner, 
similar to the use of histograms. That is, the slopes of the curves, their spread, 



"^ Krumbein, W. C, Graphic Presentation and Statistical Analysis of Sedimentary Data in Recent 
Marine Sediments, p. 564, Am. Assoc. Petroleum Geologists, 1939. 



70- 

60- 

2 40- 

S 

30- 




589 '.495 '.295 '.246 '.14 7 ' .12 4 ' .104 ' 
Grode size in mr 
HISTOGRAM 



40- 
JO- 

20- 
10- 




.495 .295 .246 .147 .124 .104 .088 .074 -.074 
Grade Size m mm 
SIMPLE FREQUENCY CURVE 





O3 = 148 




---' ' 


90- 


0| = 180 


III (well-sorted)/ 

A. 




80 - 


So=V Q./Q3 = 




7C - 








60- 


(Semiloganlhmic 


base) / 








Md 




40 - 








30 ■ 




/ °' 




20 ■ 




/ 




10- 




y 





Grade size in mm 
CUMULATIVE FREQUENCY CURVE 

Figure 80. Histogram, simple-frequency curve, 
and cumulative-frequency curve, of the Da- 
kota sandstone tabulated in table 6. 



192 



Subsurface Geologic Methods 



and the degree of asymmetry are compared qualitatively. A majority of 
workers, however, use the cumulative curves to read statistical values. Gen- 
erally three values are read — the median (Md) and the first and third 
quartiles (Qx) and { Qs) [fig- 80]. The median diameter is found by reading 
the diameter value at the point where the cumulative curve is intersected by 
the fifty-percent line. The first and third quartiles are determined as the 
diameter values corresponding to the intersections of the curve with the 25- and 
75-percent lines. Qj is assigned the larger value. 




+ 20 mesh 

+ 40 mesh 

+ 60 mesh 

+ 80 mesh 

- 80 mesh 



1 I — r 

Percent 

Figure 81. Graphic method of compihng size-analysis data of several samples. 

Applying these data, Trask '^^ has defined the geometric coefficient of 
sorting as : 

As mentioned by Krumbein, the geometric measures are essentially 
ratios between quartiles, or quartiles and median, thus eliminating both 
the size factor and the units of measurement. Trask states that if the So 
value is less than 2.5, the sediment is well sorted; if greater than 4.5, it 
is poorly sorted; and if 3.0, it has normal sorting. A study of a number 
of sandstones of the Rocky Mountain region shows these values to be too 
high. Hough ^^ points out that the coefficient of sorting for most near- 
shore marine sediments lies between 1 and 2. 

From the foregoing it can readily be seen that, if this general ap- 
proach is followed, statistical values may be recorded and compared for 
various granular clastic sediments. 

For more detailed information concerning this subject the reader is 
referred to chapter 6 of the "Manual of Sedimentary Petrography" by 
Krumbein and Pettijohn.^^ 

" Trask, P. D., Origin and Environment of Source Sediments of Petroleum, pp. 71-72, Houston, 
Tex., Gulf Publishing Co., 1932. 

"* Hough, J. L., Sediments of Buzzaris Bay, Massachusetts: Jour. Sedimentary Petrology, vol. 10 p. 
26, 1940. 

'^ Krumbein, W. C, and Pettijohn, F. J., Manual of Sedimentary Petropraphy, New York, Appleton 
Century, 1938. 



Subsurface Laboratory Methods 193 

SETTLING ANALYSIS 
L. W. LeROY 

Stratigraphic sections involving fine-grained elastics (siltstones, clay- 
stones, and shales) in most areas are difl&cult to subdivide and correlate, 
particularly if paleontologic and lithologic data are inadequate. Skeeters ^^ 
has suggested a method that is based on the rate of settling of minute par- 
ticles, through a liquid medium, which may be of some correlative value. 
This technique does not involve numerical determination of particle size 
but instead considers the settling rate of the particles and resistivity char- 
acteristics of the supernatant liquid. 

The procedure of this investigation is as follows: The argillaceous 
sample, after thorough drying, is pulverized to — 120-mesh and placed in 
a four-foot vertical glass tube (2.5 inches in diameter) into which com- 
pressed air is introduced through a stopper in the base. One hundred 
grams of material is added to 2,000 cc. of water. This mixture is then 
air-agitated for 15 minutes, after which the height of the settling mate- 
rial is measured at five-minute intervals and the results plotted. 

During the settling stage and at five-minute intervals, electrical-re- 
sistance values of the supernatant turbid liquid are measured between 
two electrodes spaced half an inch apart and suspended two feet below 
the fluid surface. 

The settling and resistivity results of several Pierre and Fox Hills 
shales are shown in figure 82. Skeeters ^*^ concluded that 

The height-of-settling-surface curves show the greatest promise of appli- 
cability to correlation. The resistance curves show considerably more similarity 
between runs on the same shale and are of sufficient variation between shales 
to offer some promise of possible value in correlation. 

STAIN ANALYSIS 
L. W. LeROY 

The application of stain solutions to polished surfaces and to thin 
sections of rocks permits the rapid identification of certain minerals and 
assists in establishing the distribution and mutual relationship of the 
minerals. 

Stain results depend on such factors as the texture and structure of 
the rock, the purity and relationships of the minerals, and the uniformity 
of the applied procedure. If more exact mineralogic determinations are 
desired, optical investigations should supplement the stain tests. 

Frequently the subsurface geologist during an examination of well 
cuttings and cores is concerned with distinguishing between aragonite, cal- 
cite (limestone), dolomite (dolostone), quartz, feldspar, and certain basic 
types of clay minerals. Some of the methods applicable for rapidly de- 

*" Skeeters, W. W., unpublished research report, Colorado School of Mines, 1942. 



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Subsurface Laboratory Methods 195 

termining these minerals and for evaluating their interrelationships in a 
rock are here briefily outlined. 

Calcite and Aragonite 

Meigen ^^ developed a method of distinguishing aragonite from cal- 
cite by immersing a polished rock surface or thin section of each for 20 
minutes in a solution of boiling cobalt nitrate and observing the resulting 
color. Aragonite stains a light purple in the initial stages but upon con- 
tinued boiling assumes a violet hue. Calcite attains a similar color 
only after several hours of immersion. For fine-grained rocks the two 
minerals are difficult to differentiate owing to the spreading of the stain. ^- 
Aragonite grains treated with cobalt nitrate, when immersed in a solution 
of ammonium sulphide, become coated with a film of black cobalt sul- 
phide. 

Calcite and Dolomite 

Several staining methods are used for distinguishing calcite from dolo- 
mite. The results of these stains differ, some being more dependable and 
exacting than others. These tests are given in the order of preference. 

Fairbanks Method ^^ — In the Fairbanks method the solution to be 
used is prepared by mixing 0.24 grams of haematoxylin, 1.6 grams of 
aluminum chloride, and 22 cc. of water and bringing the mixture to a boil; 
the solution is cooled and, after the additions of a small quantity of hydro- 
gen peroxide, filtered. Calcite, upon being immersed in the solution, 
rapidly stains dark purple, whereas dolomite remains unaffected. The ad- 
vantage of this test is its rapidity and dependability. The polished surface 
or thin section is allowed to remain in the stain o'nly thirty seconds, and is 
then removied and carefully washed in water. Boiling the rock in the stain 
solution is not required. This method is exceptionally favorable for evalu- 
ating limestone and dolostone fragments in well samples. 

Copper Nitrate Method — Calcite boiled in a concentrated solution of 
copper nitrate assumes a medium-green color; dolomite is not affected. 
The color may be fixed by immersing the sample in ammonia. This test 
is effective and yields consistent results. 

Silver Chromate Method — The polished surface or thin section is 
immersed three or four minutes in a boiling ten-percent solution of silver 
nitrate. The nitrate is then washed free from the sample, which is sub- 
sequently treated with a saturated solution of potassium chromate. Cal- 
cite and aragonite grains are stained reddish-brown; dolomite retains its 
original color. This method is accredited to Lemberg ^^ and may be con- 
sidered as giving dependable results. 

*^ Meigen, W., Eine einfach Reaktion zur Unterscheidung von Aragonit Kalkspath: Centralb f. 
Min., etc., pp. 577-578, 1901. 

^ Twenhofel, W. H., and Tyler, S. A., Methods of Study of Sediments, p. 129, New York, McGraw- 
Hill Book Company, Inc., 1941. 

^^ Fairbanks, E. E., A Modification of Lemberg's Staining Methods: Am. Mineralogist, vol. 10, pp. 
126-127, 1925. 

** Lemberg, J., Zur microchemischen Untersuchung einiger Minerale: Zeitachr, geol. Gesell., Band 
44, pp. 224-242, 1892. 



196 Subsurface Geologic Methods 

Lemberg Method ^^ — Tlie solution to be used is prepared by boiling 
for 20 to 30 minutes a mixture of 4 grams of AICI2, 6 grams of logwood, 
and 60 grams of water; the mixture is filtered and the filtrate diluted 
with 1,000—1,200 CO. of water. Calcite when immersed in the solution 
is stained light purple after five to ten minutes of boiling; dolomite 
remains unchanged. This reaction causes a film of aluminum hydroxide 
on the calcite grains and this film absorbs the logwood dye. 

Potassium Ferricyanide Method — The potassium ferricyanide method 
was developed by Heeger.^*^ It consists of immersing the rock sample first 
in a dilute solution of hydrochloric acid (1:100) containing a few drops 
of postassium ferricyanide. If the dolomite contains ferrous iron, the min- 
eral assumes a deep-blue color; calcite is not affected. This test is con- 
sidered satisfactory only if the dolomite contains ferrous iron; otherwise, 
it fails to produce results. 

Identification of Feldspars 

Twenhofel and Tyler ^^ summarized the method of distinguishing 
quartz from feldspar as follows: 

A few drops of hydrofluoric acid are placed on a thin section, or on grains 
mounted in Canada balsam with their upper surfaces exposed, and allowed 
to remain one or two minutes before being gently washed off. The acid produces 
a thin, gelatinous film of aluminum fluorosilicate on the feldspar and other 
aluminous minerals but leaves the quartz clear. After washing, the specimen 
is immersed in a water-soluble organic dye for about five minutes and then 
again washed. Fuchsine, methylene blue, safranine, or malachite green may 
be used as a stain. . . . The depth of color retained on staining is greatest 
with anorthite; becomes successively lighter with less calcic feldspars; and 
is lightest with orthoclase or microcline. 

The degree of staining is improved if the grains are exposed to the 
fumes of hydrofluoric acid for two or three minutes. Care should be exer- 
cized in washing after staining, as the stain is easily removed from the 
corroded feldspar grains. 

According to Gabriel and Cox,^^ the potash feldspars may be iden- 
tified by exposing the rock to hydrofluoric-acid fumes and then staining 
it with a diluted solution of sodium cobalt nitrite, which is prepared by 
adding 15 cc. of glacial acetic acid and 25 cc. of water to 12.5 grams of 
Co(N03)2.6H20 and 20 grams of NaNOs. The potash feldspars assume 
a strong yellow color from the formation of potassium cobalt nitrite. 
Quartz and plagioclase grains are not affected. 

Potash feldspars (orthoclase, microcline) may be differentiated from 
the calcic plagioclase feldspars (laboradorite, bytownite, and anorthite) 

*° Lemberg, J., Zut microchemischen Untersuchung von Calcit, Dolomit, un Predazzpit: Zeitschr. CeoL 
Gesell., Band 39, pp. 489-492, 1887. 

** Heeger, J. E., Ueber die Mikrochemische Untersuchung jein verteiler Carbonate im Gesteinssckliff : 
Centralbl. Mineralogie 1913, pp. 44-51, 1913. 

S' Twenhofel, W. H., and Tyler, S. A., op. cit., p. 131. 

*^ Gabriel, A., and Cox, E. P., A Staining Method for the Quantitative Determination of Certain Rock 
Minerals: Am. Mineralogist, vol. 14, pp. 290-292, 1929. 



Subsurface Laboratory Methods 197 

by the followftig procedure: (1) Pulverize the sample or disaggregate the 
sandstone to minus-SO-mesh; (2) boil the material for one minute in 
hydrofluoric acid; (3) wash the acid-treated sample gently in distilled 
water; (4) boil the washed sample for ten minutes in a water-saturated 
solution of eosine y; and (5) carefully wash the sample and remove the 
excess dye solution. 

The plagioclase grains are coated with a medium- to dark- to 
orange-red film. Orthoclase and quartz are not stained. The combination 
of this test and the cobalt-nitrite test for orthoclase serves as a basis for 
rapidly estimating the feldspathic content of sands and sandstones. The 
sodic feldspars (albite, oligoclase, and andesine) are not noticeably af- 
fected by the eosine test. 

Eosine dye may be used to identify nephelite and cancrinite (ortho- 
silicates) . Nephelite assumes a light-pink discoloration, whereas can- 
crinite attains a much darker pink. The discoloration is produced within 
the grain and not as an exterior film as with the calcic plagioclases. 
Sodalite is not affected by the dye. 

Clay-Mineral Stain Tests 

In recent years considerable work has been devoted to clay miner- 
alogy. Mineralogic analyses have indicated three important groups of 
the clay minerals (kaolinite, montmorillonite, and illite). Identification 
of these clay-mineral species within each group is extremely difficult owing 
to their minute size. Chemical, optical, X-ray, electron-diffraction, and 
differential thermal-dehydration methods are required for precise deter- 
mination. 

Several dye tests are employed for assisting in differentiating vari- 
ous clay groups. Extreme care should be exercised in applying these tests, 
for results may be extremely variable because of impurities, complex 
mineralogic associations, and inconsistent preparation procedure. The 
stain results may be observed in reflected light under either a petrographic 
or binocular microscope at magnifications from 30 to 120 diameters. 

Benzidine Test — A saturated water solution of the organic compound 
benzidine (or benzidine hydrochloride) produces a blue coloration in 
contact with clay minerals of the montmorillonite and illite groups, al- 
though the benzidine solution itself is slightly pink. The sample is not 
treated with hydrochloric acid prior to application of the stain solution. 
It has been reported that manganese dioxide and organic matter may 
cause formation of a blue coloration in the absence of bentonite, and that 
ferrous iron or other reducing agents may prevent the development of 
coloration. ^^ Gypsum has a pronounced effect on the benzidine test. This 
effect may be minimized by first boiling the material in water, pouring 
off the fine fraction, thoroughly drying it at 105° C, and then applying 
the stain solution. 



^' McConnell, Duncan, Notes on Properties and Testing of Bentonites: U. S. Bur. Reclamation, 
Denver, Laboratory Kept. Pet-44B, 1946. 



198 Subsurface Geologic Methods 

Crystal-Violet Test — The crystal-violet dye solution (25 cc. of nitro- 
benzene, 0.1 gram of crystal violet) causes acid-treated montmorillonite 
first to stain green and then greenish yellow or orange yellow. lUite as- 
sumes a rather dark-green color. Kaolinite merely absorbs the violet stain. 

Safranine y Test — Another stain applicable for identifying clays of 
the montmorillonite and illite groups is the safranine y (nitrobenzene 
saturated with safranine y) . McConnell ^^ summarizes this test as follows: 

(1) A small representative sample (about 20 grams) is selected, crushed, 
and placed in a beaker; (2) strong hydrochloric acid is added in amounts 
four or five times the volume of earth material. If significant amounts of 
carbonates are present the quantity of acid is proportionally increased. The 
sample in acid is retained at elevated temperatures for an hour or two. Suitable 
temperatures can be obtained by placing the sample on top of a small labora- 
tory oven; (3) the acid-treated sample is washed five times, using 200 milli- 
liters of distilled water for each washing. The earth material is then trans- 
ferred to a filter paper, which is placed in a dish and oven-dried at about 
105° C. ; (4) the dried material is examined and one or more samples are 
removed from the filter paper for staining. Considerable care must be exer- 
cised in the selection of this sample (or samples) because stratification invar- 
iably takes place in the funnel during washing; (5) three or four drops of 
nitrobenzene saturated with safranine y are added to the mineral powder and 
the quantities of colorless, red, purple, and blue grains are estimated. The 
quantity of blue and purple grains compared with the total number is an 
indication of the amount of bentonitic material present. 

This test is apparently capable of giving anomalous results in rare 
instances but is probably subject to interferences no more frequently than 
the benzidine test. 

Kaolinite is unaffected by safranine y. Minerals of the montmoril- 
lonite group become blue when the dye is applied, whereas illite grains 
tend to exhibit a more bluish-purple to purplish hue. 

Malachite-Green Test — After being acidized with hydrochloric acid 
the clay minerals of the kaolinite group, when in contact with malachite- 
green solution (25 cc. of nitrobenzene, 0.1 gram of malachite green), 
become a bright apple-green. The montmorillonite and illite minerals 
commonly become pale yellow or greenish yellow. 

Summary of Clay-Stain Results 

In table 7 results of clay-stain tests are given in summary. 
For favorable results in clay-stain tests the following precautions 
should be observed. 

1. The acidization (HCl) procedure should be complete and uni- 
form. Best results are obtained if the material is pulverized and passed 
through a 200-mesh screen. 

2. After step (1), the sample should be thoroughly washed free of 
the acid with distilled water; otherwise, consistent stain results cannot be 
obtained. 



' McConnell, Duncan, op. clt. 



Subsurface Laboratory Methods 



199 



3. After step (2) , the acidized material must be completely dehy- 
drated by drying for several hours at temperatures of about 105 ° C. 

4. About one milligram of the sample material should be used when 
the stain solutions are applied. 

5. The treated sample should remain in the stain solution for five 
minutes in order to obtain the best coloration results. 

6. The reflected light source should be controlled. 

Clay mineralogy offers possibilities for serving as a means of corre- 
lating and subdividing homogeneous argillaceous and carbonate sections. 
The latter rock types involve insoluble residues. Clay mineralogy can 
also be useful in evaluating changes in the porosity and permeability of 
sands and sandstones. 

The presence of montmorillonite clay types is extremely detrimental 

TABLE 7 
Summary of Clay-Stain Results 



Mineral 
group 


Safranine "y" 
(acidized 
sample) 


Malachite 

green 
(acidized 
sample) 


Crystal violet 
(acidized sample) 


Benzidine 

(unacidized 

sample) 


Kaolinite 

Montmoril- 
lonite 

Illite 


Red 
Blue 

Bluish purple 
to purple 


Green 

Yellow to 
greenish 
yellow- 
Yellow 


Violet 

Yellow, greenish 
yellow, or orange 
yellow 

Dark green 


No reaction 

Blue (variable 
hues) 



in many engineering projects because of their swelling properties. Sedi- 
ments containing these minerals should be completely analyzed before 
construction of buildings, highways, dams, and other projects in order 
to predict the reaction of the earth materials. 

Shape Analysis 
L. W. LeROY 

Various procedures are followed in determining the sphericity, round- 
ness, and flatness values of sedimentary particles. 

Krumbein and Pettijohn^^ give the following factors controlling the 
shape of sedimentary grains and fragments: (1) the original shape of the 
fragment; (2) the structure of the fragment, as cleavage or bedding; (3) 
the durability of the material; (4) the nature of the geologic agent; (5) 
the nature of action to which the fragment is subjected and the violence 
of that action (rigor) ; and (6) the time or distance through which the 
action is extended. 

Varying degrees of sphericity ("measured by the ratio of 5/5, where 

°' Krumbein, W. C, and Pettijohn, F. J., Manual of Sedimentary Petrography, p. 278, New York, 
D. Appleton-Century Co., 1938. 



200 



Subsurface Geologic Methods 



s is the surface area of a sphere of the same volume as the fragment, and S 
is the actual area of the object. For a sphere the ratio is 1. For all other 
solids the ratio has a value less than one") and roundness (a measure of 
the angularity of the edges and corners) of detrital grains have served in 
correlating certain strata. In the Rangely oil field of northwestern Colo- 
rado and adjacent areas, the Entrada and Navajo (Jurassic) sandstones 
are differentiated from adjacent lithic units by the rounded and frosted 
character of the quartz grains. 

Rittenhouse ^- has used the degree of roundness of tourmaline and 
zircon in correlating various strata in the Appalachian Basin. He states: 

In the Appalachian Basin roundness of heavy minerals is extremely 
valuable as a criterion for differentiating various Mississippian and Pennsylva- 
nian oil and gas sands, for outlining petrographic provinces, and for inter- 




FiGURE 83. Measurements of pebbles re- 
quired in determining sphericity, round- 
ness, and flatness values; a and b are 
determined from maximum image 
orientation; c value is normal to a 
and b. 



preting geologic history. Roundness is particularly significant in the basin 
because fossils are rare and the heavy-mineral suite is restricted. . . . 

Petti John comments :^^ 

The roundness of a clastic particle sums up its abrasion history. Spher- 
icity, on the other hand, more largely reflects the conditions of deposition at 
the moment of accumulation, though to a more limited extent sphericity is 
modified by the abrasion processes. 

According to Fraser,^^ the absolute size of the grain, nonuniformity 
in the size of the grain, the proportions of various sizes of grains, and the 
shape of the grain control porosity of unconsolidated deposits. He fur- 
ther states: 

Regularities in shape should result in a larger possible range in porosity, 
as irregular forms may theoretically be packed either more tightly or more 
loosely than spheres. The degree of rounding generally varies for different 

^" Rittenhouse, Gordon, Grain Roundness — A Valuable Geologic Tool: Am. Assoc, Petroleum Geolo- 
gists Bull., vol. 30, no. 7, pp. 1192-97, July 1946. 

"2 Pettijohn, F. J., Sedimentary Rocks, p. 53, New York, Harper and Brothers, 1949. 

°* Fraser, H. J., Experimental Study of the Porosity and Permeability of Clastic Sediments: Jour. 
Geology, vol. 43, no. 8, pp. 910-1010, 1935. 



Subsurface Laboratory Methods 



201 



grain ^zes in any natural deposit, because of differences in the mineralogical 
composition of different grades. ... It is difficult to determine the effect of 
shape of grain on porosity, because of the difficulty of obtaining angular par- 
ticles of the same size. . . . Angularity may either increase or decrease 
porosity; most often it increases porosity. The only type of "angularity" found 
to cause a decrease in porosity is that in which the grains are mildly and uni- 
formly disk-shaped. 

Factors affecting permeability (in addition to temperature, hydraulic 
gradient, and coefficient of permeability) include uniformity and range 
of grain-size, shape of grain, nature and uniformity of packing, surface 
conditions of the grains, stratification, consolidation, and cementation of 
the material. ^^ 

Krumbein ^^ has given an interesting discussion on determining spher- 



fi5 



1 
.9 

.8 
.7 
.6 

.5 

I 

.4 
• 3 
.2 

.1 



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zzzhzhl:f;^zzzhzzzzzzzz<iizzzz[zzzz'iiizzz::zzz: 


\\m*miv^ m ^ffl-I 1 TH-Ll \W 


::::t::^r:+:^5::::::^::::i::::=^;::::::::::;5:;: 


::::i:::5-::;:v::::::^s-::::::: :^=:;::::::::::: 


1^1 rk 11h^4 Irtili L]J 


^y N f^^^ ^i^ iTTW-gg 


::|::^:;::fs;:::::::$::::::±:::E>;:::::::::;;::: 




RffittSflMfflwftftrrrnfl H THtMJ m 


^a. — 2 41— -^ 5^ ^ =. 


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:::d±::::5+::i:::hs-::::::::i::-i:;:::::::::: 


1 a PKJ \\mll\ TtittH-k^ 


l_i-_\ Us- — 1 *»z-l - = 


:|::::5-:::::::^5;_:::::::::::::M:;::::"::::::: 




ffl PwlB^ 


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t- - 

±_ :_ e: :: ±_-: 



.1 



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.4 .5 
c/b 

Figure 84. Chart for determining sphericity of pebbles; a, b, and c, values represent 
the long, intermediate, and short dimensions of the pebble. (From Krumbein, 
Jour. Sedimentary Petrology.) 

"^ Fraser, H. J., op. cit., p. 959. 

Krumbein, W. C, Measurement and Geological Significance of Shape and Roundness of Sedimen- 
tary Particles: Jour. Sedimentary Petrology, vol. 11, no. 2, pp. 64-72, Aug. 1941. 



202 Subsurface Geologic Methods 

icity values of pebbles. Three-dimensional values are recorded, maxi- 
mum (a), intermediate (6), and minimum (c) (fig. 83). The ratios b:a 
and c:b are calculated. From these ratios the sphericity index is obtained 
from a control chart (fig. 84). 

Roundness values of grains and pebbles as determined by Wadell ^^ 
are computed from a maximum-plane image (projected cross section) in 
which the summation of the radius of the individual corners is divided 
by the number of corners and this value divided by the radius of the max- 
imum-inscribed circle. Roundness is expressed by the formula: 

where r is the radius of each corner, R is the radius of the maximum 
inscribed circle, A^ is the number of corners, and P is the degree of round- 
ness. Roundness values may also be obtained from a chart. Grains may 
have the same roundness but varying sphericities, whereas other grains 
may have the same sphericity but varying roundness. 

The flatness ratio ^^ of pebbles and grains is expressed by the form- 
ula: 

It 

F=(length+width-^twice the thickness) =- - 

The combination of sphericity, roundness, and flatness values permits 
quantitative expression of the shape characteristics of grains and pebbles. 

ELECTRON-MICROSCOPIC ANALYSIS— SOME GEOLOGIC 
APPLICATIONS IN CORRELATION WORK 

CARL A. MOORE 

Ability to discriminate among minute objects that lie very close to- 
gether is described as the resolving power of a microscope. In spite of 
various methods that may be employed to increase the resolving power of 
a light microscope, objects separated by less than 0.1 micron (0.0001 milli- 
meter) cannot be resolved. Thus it is that the limits of the light micro- 
scope are not the lack of skill on the part of the designer but rather are 
due to the light — ^the media used for observation. 

This limitation of the light microscope is an important factor in 
microscopy; for example, the study of viruses must be conducted with par- 
ticles and separations much smaller than this, and the study of colloids 
necessitates greater resolving power than that possible with the light micro- 
scope. 

With the introduction of the electron microscope, this limit on resolv- 
ing power has been greatly decreased, since the magnification is no longer 

®' Wadell, Hakon, Sphericity and Roundness oj Rock Particles: Jour. Geology, vol. 41, pp. 310-331, 
1933. 

"S Wentworth, C. K., The Shapes of Beach Pebbles: U. S. Geol. Survey Frof. Paper 121-C, pp. 
75-83, 1922. 



Subsurface Laboratory Methods 



203 



limited by the wave length of visible light. Theoretically, the electron 
microscope should be capable of resolving powers as small as atomic di- 
mensions. In actual practice, however, the microscope has not been per- 
fected to that extent. Nevertheless magnifications of over 100,000 diame- 
ters are practical with the electron microscope, as compared with a useful 
limit of 2,000 diameters for the light microscope. 

Description of the Microscope 

Figure 85 is a comparison of the optical microscope with the electron 
microscope showing equivalent parts: magnetic fields are equivalent to 
lenses; both have specimen levels; and both have photographic plates for 



OPTICAL 
MICROSCOPE 



ELECTRON 
MICROSCOPE 



Photographic Piote 



Eyepiece or 

Photo -projector 




Obiective Lens System 



Substoge Condenser 



Reflector 





Projected » 
Innage 




Phologrophic Plate 



Magnetic Coil 
Projector 



''^ZZZ>,' 



Mognetic Coil 
Serving as 
Objective 

Magnetic Coil 

Concenlrotmg 
Electron Beom 



- • C Source 
Light' ^E 



A, 



Figure 85. Comparison of light microscope and magnetic electron microscope. 
(From Burton and Kohl.) 



pictures. For comparison, the electron microscope is diagrammed upside 
down. 

A simplified drawing of the R.C.A. compound magnetic electron mi- 
croscope, type EMB, is shown in figure 86. Focusing is accomplished by 
varying the lens power. The specimen mount is the movable stage. As the 
stage is inside the vacuum portion of the microscope, it is moved by means 
of fine screws and a metal flexible bellows. 

The electron beam is concentrated on the specimen by the magnetic 
field produced in the condenser-lens coil. After passing through the speci- 
men, the electrons are focused by the objective-lens coil into an intermedi- 



204 



Subsurface Geologic Methods 



ate image, and the projection-lens coil produces a further magnified 
image on the fluorescent screen in the final viewing chamber. 

To facilitate the initial adjustment of the specimen, a port is provided 
for viewing the intermediate image on a fluorescent screen close to the 



Electron Source 



Condenser Coil 



Specimen Mount 
Objective Coil — 



Port for Intermediate Viewing 



Projection Coil- 



Ports for Viewing Final Enlarged Image 
Fluorescent Screen or Photograptiic Plate 



Vacuum Pump 




Figure 86. Simplified drawing of electron microscope, taken from "Electron Micro- 
scope" prepared by R.C.A. Manufacturing Company, Camden, N. J. 

plane of the projection-lens coil. By virtue of the relatively low magni- 
fication at this point, it is possible to select the most interesting part of 
the specimen and to move it into position to be magnified further by the 
projection-lens coil. 

Six observation windows enable a number of spectators to view the 
image simultaneously. With the choice of a selected field of view and the 
magnification adjusted to the desired value, the fluorescent screen is raised, 



Subsurface Laboratory Methods 205 

exposing a photographic plate to the electrons. This plate is carried in a 
holder in the vacuum system of the microscope. Magnifications of 1,000 
to 20,000 diameters are possible, and the definition of the photograph is 
sujfficiently clear to allow further optical enlargement to full useful mag- 
nification. 

Specimen-Mounting Techniques 

It was necessary to devise a special specimen-mounting technique in 
order to work with the small areas that are enlarged to full magnifications 
for study. Most specimens are mounted on a 400-mesh screen. This screen 
is dipped into a solution of collodion, which dries quickly, leaving a 
strong film approximately one micron (0.001 mm.) thick between the in- 
dividual wires. 

The material for study may be placed on this collodion film in one of 
several ways: (1) manually, under high-power binoculars; (2) precipi- 
tated from solution onto the screen; (3) by passing the screen coated 
with collodion through a culture of the material; and (4) by placing a 
drop of material suspended in a liquid onto the screen. 

No wet or living tissue can withstand the high vacuum of 10"* to 10'^ 
millimeters of mercury in the electron microscope. However, the micro- 
scope is being used extensively in biological studies on materials ranging 
in size from that of the organs of animals and insects downward through 
that of the bacteria and of the viruses and even of large molecules. The 
material in turn must be thin enough to allow the passage of electrons 
through it. Some materials deteriorate when subjected to the intense elec- 
tron bombardment, and some materials may heat up during this bombard- 
ment. Owing to the high vacuum, this heat cannot be transmitted or con- 
ducted away from the subject. 

Possible Uses in Correlation Work 

The usefulness of any method of correlation lies in its ability to 
indicate or prove the existence of equitable or similar ages or environ- 
ments of deposition between two areas, two wells, or two geologic out- 
crops. Most methods in geology originally included only the megascopic 
aspects: for example, similar or identical fossils and equivalent succes- 
sions of beds, to mention two. With the advances in geologic techniques, 
more precise correlation has been possible by utilizing microscopic sim- 
ilarities for correlations, as in micropaleontology, sedimentary petrology, 
and microlithology. 

With the electron microscope, it should be possible to achieve the 
ultimate in utilizing submicroscopic similarities for correlations. Some 
uses possibly peculiar to this microscope are herein listed and discussed. 

Bed Identification 

It is possible that many minute similarities exist in beds or forma- 
tions, which, if they could be seen and studied, could be used to correlate 



206 Subsurface Geologic Methods 

subsurface beds. In studying sandstones, for example, it would not be 
possible to observe the actual sand grains, but it would be necessary to 
study the cementing material and any foreign material in the sandstone. 
Thus, in investigating the clay content versus water conductivity of oil 
sands. Bates, Gruver, and Yuster ^^ isolated mica crystals and photo- 
graphed them in the electron microscope for study. Sandstones bearing 
similar mica crystals might be correlative, if other criteria attested to a 
possible correlation. 

Correlations with limestones should involve a different set of condi- 
tions. As a general rule limestones are compact or, if porous, contain 






Figure 87. Electron-microscope picture of Attapulgus clay (X 20,000). Note minute 
fibers and bundles of fibers, with very few larger grains. These average less than 
^M- (=0.000125 mm.) in diameter and are of colloid size. Courtesy R.C.A. 
Laboratories and Standard Oil Development Company.) 

comparatively large pores and openings. It would be difficult to impos- 
sible to grind a thin section of limestone to a thickness allowing the 
electrons to pass through the specimen and produce an image on the photo- 
graphic plate. The pores of the limestone are so large as to preclude any 
precise study of their contour or shape. For these reasons, a possible ap- 
proach would lie in the study of the residues after the limestone had been 
dissolved in some suitable solvent. Either the filtrates could be examined 
for correlatable objects, or the residue, which is often largely clay, might 

^ Bates, T. F., Gruver, R. M., asd Yuster, S. T., Influence of Clay Content on Water Conductivity 
of Oil Sands: Oil Weekly, Oct. 21, 1946. 



Subsurface Laboratory Methods 



207 



lend itself to study in the electron microscope. This latter case leads into 
the problem of the study of shales. 

Correlation of Clays 

Quoting from Hillier,^^*^ ". . . particles of various types of clay 
have probably been subjected to more examination by means of the elec- 
tron microscope than any other type of material." Some clays are com- 
posed of grains of about fifty angstroms in thickness and a few ang- 
stroms wide. Studies of the nature and correlation of such minute 




Figure 88. Electron-microscope picture of infusorial earth (X 20,000) sold by 
Central Scientific Company, Chicago. Note that fibers and bundles of fibers are 
very similar to those in photograph of Attapulgus clay in figure 87. Diatom 
fragment near center of photograph is about l%.u in length by l|.i wide. Openings 
in shell are less than % [j, in diameter (0.0002 mm.) and would barely be dis- 
cernible in the light microscope. (Courtesy R.C.A. Laboratories and Standard 
Oil Development Company.) 

particles in the electron microscope are dependent on characteristic shapes 
and not on chemical combinations. 

One of the clays used extensively in laboratories and refineries for 
filtering is called "Attapulgus clay," so named for Attapulgus, Georgia. 
Chemical analyses of this clay show it to be chiefly montmorillonite, a 
hydrous aluminum silicate, but the individual microcrystalline masses 
cannot be identified or resolved under the best light microscope. Figure 
87 is an electron-microscope picture of this clay, X 20,000, showing an 

'™ Hillier, J., Electron Microscopy: Am. Ceramic Soc. Bull., Nov. 1946. 



208 Subsurface Geologic Methods 

abundance of masses of minute fibers. These fibers are the so-called micro- 
crystalline masses that cannot be identified or resolved under the polar- 
izing microscope. 

Infusorial earth is described as a "siliceous earth made up largely of 
siliceous fragments of Infusoria, used as fulling material and as a filter- 
ing and absorbing agent." Figure 88 is an electron-microscope picture of 
this material, X 20,000, showing fibers very similar to the Attapulgus clay. 
The similar shapes of constituent parts of these two materials attest to their 
similar physical properties. 

Clays might lend themselves to study and correlation in the electron 
microscope in the following ways: 

1. The submicroscopic mineralogy and crystallography of clays 
might be studied. Minute crystals of rutile have been identified in titanium- 
rich clays. These crystals were too small to be identified under a light 
microscope. Detailed studies should bring out several similar instances 
of submicroscopic mineralogy that could be of value in correlation. 

2. The presence of submicroscopic organic forms too small to be 
identified or even noted under a light microscope could provide the 
means for bed identification. This would involve the development of, 
shall we say, "electron micropaleontology," wherein organic forms far 
below the smallest fossil known would be studied, 

3. Structural details of clays, pertaining to possible physical and 
physicochemical properties of the clays, should lend themselves to study 
in the electron microscope. The importance of this point might be stressed 
by suggesting that the electron microscope is believed to be capable of 
resolving giant molecules. At these particle sizes the physical and chem- 
ical properties would be dependent one upon the other and should be diffi- 
cult to separate. 

4. In the same general way, clay residues from limestones might be 
studied. Identification would depend upon the structural details and per- 
haps the mineralogy. Chemical or spectrographic methods of study would 
probably be of more value here than would the electron microscope. 

5. Physical studies of the response of clays to the high vacuum in 
the electron microscope and changes due to the electron bombardment, 
with subsequent heating of the samples, might yield significant similari- 
ties and diff'erences of correlative value. 

Long-Range Correlation 

The foregoing discussion has involved detailed correlations between 
individual beds. It was pointed out that electron-microscope techniques 
are not in general use as yet and may not be used except in unusual cases. 
Long-range correlations, of course, depend upon equivalent criteria being 
found over long distances. For this reason, long-range correlations with 
the electron microscope are subject to the same considerations as were the 
closer, detailed correlations. 



Subsurface Laboratory Methods 209 

Studies of Crude Oil 

It will be necessary to develop a technique for studying crude oils in 
the electron microscope. In one study a specimen of crude oil was mounted 
in the usual manner on collodion film, and a monotonous gray field was 
seen, except for one object or group of objects (fig. 89) . This was com- 
posed of a number of oval bodies; some of these are seen to be solid, 
while others appear to be breaking up. It is possible that this object was 
not able to withstand the high vacuum and electron bombardment in the 
electron microscope, and the photograph caught the material in the process 
of disintegration. 





Figure 89. Electron-microscope picture (X 20,000) of object found in sample of 
crude oil from Athabaska tar sands in Alberta. Note dark, oval bodies associ- 
ated with two somewhat larger, circular bodies. These oval bodies may be 
spores or minute protozoan tests. (Courtesy R.C.A. Laboratories and Standard 
Oil Development Company.) 

A possible approach to the study of crude oils in the electron micro- 
scope may be as follows: 

1. All foreign substances in the oil and in the extracts that are 
possible to prepare for study in the microscope would be studied. 

2. Bacteria in the oils and in the extracts would be observed and 
identified. 

3. The nature of coloring material in some of the darker oils would 
be studied. Is color due chiefly to the presence of foreign materials, or 
could it be due to molecular combinations? 



210 



Subsurface Geologic Methods 



4. The behavior of the oil and extracts during preparation would 
be studied, and the reactions to the high vacuums and to the electron 
bombardment observed. 

5. The R.C.A. engineers and research physicists have photographed 
what they believe to be giant molecules in the electron microscope. Mole- 
cules of crude oil are believed to be disposed in some sort of regular pat- 
tern and may be quite large. Perhaps actual molecular differences may 
be found in crude oils, in the extracts, or in the various fractions that 
may be used for correlation. 

Paleontologic Studies 

Generally speaking, paleontologic specimens are too large for study 
in the electron microscope. It should be valuable, however, in studying 





A 



B 



Figure 90. A — Comparison of photograph of diatom shell under light 
microscope (left) and electron microscope (right) (X 5,000). 
The light-microscope picture indicates presence of rows of holes, 
but the electron microscope shows size and arrangement of 
these rows of holes. Holes are approximately 0.5fx in diameter, 
separated in the row by a distance of 0.2\i. The rows themselves 
are approximately 0.9^ apart. B — Corresponding photographs of 
diatom shells as in A. Light-microscope picture (left) shows 
bars in shell and hints at presence of openings in the slots. 
Electron-microscope picture (right) shows clearly the small 
holes approximately 0.14(j, in diameter. Rows are about 0.2n 
apart. (Taken from Burton and Kohl.) 

details of fossils too small for study under a light microscope, such as 
diatoms, spores, algae, and some protozoans. A comparison of diatom 
shells photographed with a light microscope and the electron microscope 
(fig. 90) shows that under the light microscope the number and arrange- 
ment of the perforations can hardly be determined, while the electron 
microscope indicates clearly the detail, arrangement, and number of 
perforations. 

Conclusion 

The electron microscope has opened up a new realm of research and 
endeavor. It is being adapted to a great number of scientific fields both 
for research and for industrial purposes. Future developments should 



Subsurface Laboratory Methods 211 

increase the resolving power far beyond the best that is available today, 
but, conversely, this increase in resolving power will be one of the limit- 
ing factors of the microscope, because by working with very minute objects 
it is not possible to mount particular specimens for study. Most geologic 
techniques do not require these extremely high magnifications, however, 
and things geologic are usually too large for these magnifications. Fur- 
ther research in strictly geologic fields will have to be limited to par- 
ticular problems where the microscope can be fully utilized, 

X-RAY ANALYSIS 

N. CYRIL SCHIELTZ 

Today we have developed into the greatest industrial nation in the 
world with the highest standard of living. This is obviously because, as 
a nation, we have been able to develop and to accept new scientific 
methods and tools. Nevertheless, our progress has been greatly retarded 
because we failed on many occasions to make use of new developments 
as soon as they were available. This is true particularly of the X-ray- 
diffraction techniques. Over three and a half decades have elapsed since 
Laue discovered that X-rays interact with crystalline materials to give 
diffraction effects; yet a surprisingly great proportion of our scientific 
and administrative personnel, such as engineers, chemists, and geologists, 
have so little knowledge concerning it that they are unaware of the possi- 
bilities that the method offers, especially as a research tool. As a conse- 
quence, many problems have gone unsolved or have required an excessive 
amount of time and effort before a solution was obtained. Obviously, this 
regrettable situation exists, at least in part, because, although the technique 
required to make the X-ray patterns is relatively simple, rather specialized 
knowledge and considerable experience are essential before one is able to 
interpret the data properly. Even today many industries fail to appre- 
ciate this fact and are attempting to undertake X-ray-diffraction studies 
with personnel whose training is entirely inadequate to obtain satisfactory 
results. As a consequence, this otherwise powerful research tool some- 
times is soon grossly neglected or abandoned because the returns do not 
justify the cost of installation and operation. In reality X-ray-diflfraction 
studies have contributed a vast amount of valuable information to indus- 
try and research; however, most of it has come from the laboratories of 
our educational institutions, federal agencies, and a few large industries. 

Since this discussion is directed principally toward a reading audi- 
ence which may have only a limited acquaintance with the method, a brief 
discussion concerning the mechanism of diffraction appears desirable. All 
crystalline matter is composed of atoms or molecules arranged in such^a 
manner that they form definite families of planes in various directions 
through the crystal. By considering primary X-rays to be reflected by 
these planes in the face of the crystal, the Braggs were able to reduce 



212 



Subsurface Geologic Methods 



Laue's original mathematically complex analysis of this interaction be- 
tween X-rays and crystalline matter to terms of great simplicity. In 
figure 91 two such planes AB and CD represent one of the many families 
of planes found in a crystal. Two rays emf and gnoph of the defined 
X-ray beam are shown to be partly reflected from these planes when 
striking them with an incident and reflected angle of 9. According to the 
laws of optics these reflected rays must be in phase to be observed as a 
reflection. Consequently, ray gnoph must be longer than ray emf by an 
integral value of the wave length A. Inspection reveals that this path 
diff"erence is the distance nop and that no=d sin 6, and op=d sin 9 ; thus 
nop=2d sin 9=nX, which is the statement of Bragg's law. 

Although this equation is satisfactory for calculating diff"raction 
eff"ects, it nevertheless reveals little of the actual diff"raction mechanism 




B 



D 



Figure 91. Reflection of X-ray beam from planes in face of crystal. 



involved. A reasonable understanding of this mechanism can be gained 
from the familiar two-dimensional analogy of the interaction of waves on 
water. Figure 92 shows in successive steps (1) the generation of a circular 
set of waves from a series of parallel wave fronts by a post (or other 
small object) in a quiet body of water; (2) the interaction of these newly 
generated circular waves from a row of equally spaced posts produced 
new diffracted wave fronts; (3) the interaction of these generated circular 
waves from two rows of posts (two planes) under conditions where Bragg's 
law is not satisfied; and finally (4) the interaction of these waves where 
the angle 9 has been so chosen that all conditions for the observance of 
diffraction eff"ects by this particular family of planes have been satisfied. 
The fact that the diff'racted wave fronts from each row of posts are one- 
quarter of a wave length out of phase with those diff'racted by the adjacent 
rows, under the conditions where Bragg's law is not satisfied, immediately 
shows that we cannot observe any diff'raction from this family of planes 
under the selected conditions. On the other hand, when the angle 6 has 
been so adjusted that Bragg's law is satisfied, all of these wave fronts 



Subsurface Laboratory Methods 213 

coincide: that is, they are in phase and diflfraction effects from this par- 
ticular family of planes are observed. A sketch set into the figure shows 
how this phenomenon is related to conditions in the X-ray camera. 

This simple two-dimensional analogy can be applied to the three- 
dimensional diffraction of X-rays by crystalline matter if the posts are 
replaced by a regular assemblage of points (atoms or ions) distributed 
in space at a distance that is of the same order of magnitude as the wave 
lengths of X-rays. Spherical waves are created when X-rays, which are 
electromagnetic waves, cause forced oscillations of the planetary elec- 
trons of the atoms which they traverse, the electrons absorbing energy 
from the X-rays when moving away from the nucleus and radiating energy 
in all directions when moving toward the nucleus. Inspection reveals that 
this three-dimensional point system will produce very narrow pencils of 
rays only in those directions in which these spherical waves are in phase. 
These reinforced waves are the rays that produce the individual spots in 
X-ray patterns (Laue, rotation, Weissenberg, etc.) obtained from single 
crystals. If the single crystal is replaced by a large number of smaller 
crystals, that is, a powder, the 29 angle with the undiffracted beam must 
remain constant since, in Bragg's equation, d for the particular set of 
planes and the wave length. A, of the X-rays from a particular target 
material are fixed. The crystals of the powder with their statistical 
orientation, unless preferred orientation effects result owing to peculiar 
crystal shapes, then must produce a whole series of such discrete pencils, 
so that as a result a continuous diffraction cone with an apex angle of 4^ 
is obtained. If this cone is now recorded on a photographic film placed 
perpendicular to the cone axis, the diffraction effect is obtained as a line 
which is in the form of a ring. A pattern on which the diffraction rings 
from all families of planes have been recorded is usually referred to as a 
powder pattern and consists of a series of concentric rings on a flat film, 
or arcs of rings on a cylindrical strip of film. 

Figure 92 reveals that a fixed space arrangement of atoms with 
definite fixed distances between them must always produce precisely the 
same X-ray pattern. Furthermore, if the same space arrangement is re- 
tained but the distances between atom centers are changed,^ the X-ray 
pattern will retain its same general appearance but will either expand or 
contract. On the other hand, if the space arrangement is altered, the 
pattern is changed. Consequently, X-ray-diffraction patterns are a sort 
of fingerprint of crystalline materials. Each individual substance present 
in a mixture will produce its unique diffraction effects, so that the pattern 
derived from the mixture is a composite of the patterns of all the materials 
or compounds in the mixture. Furthermore, the intensities of the lines of 
the individual patterns are a function of the relative amount of the mate- 
rial present in the mixture, so that the method also has quantitative aspects. 

'Atomic diameters vary from one element to the next; that is, the silicon atom as an ion has a dia- 
meter of 0.8 angstroms (lA ^ lO"* cm.), calcium 2.0 A., potassium 2.66 A., etc. 




ta 



Subsurface Laboratory Methods 215 

Scope 

A complete discussion of the various methods of recording diffrac- 
tion patterns is obviously beyond the scope of this section, and the reader 
is referred to the original papers and standard texts.^ 3 4 5 6 7 'p}^jg 
discussion will be limited to the information required by geologists for 
the identification of geologic materials. Such information includes the 
advantages and disadvantages of the X-ray-diffraction method; the funda- 
mentals of recording the data with apparatus employing photographic 
film or Geiger-counter circuits; the selection and preparation of the mate- 
rial to be investigated; the selection and processing of the films; the con- 
version of the data into usable form; the interpretation of the data; and 
when the method can be applied advantageously to geologic problems. 

The interpretation of the data requires (1) the conversion of the 
lines in the X-ray powder pattern to their corresponding interplanar dis- 
tances so that Hanawalt's ^ method employing the card file of X-ray- 
diffraction data® can be used; or (2) an extended series of standard 
patterns which are used for direct comparison if complementary data, such 
as optical measurements, are available to limit the unknown to a work- 
able number of possible materials; or finally (3) the application of the 
reciprocal lattice to make use of unit-cell data, if powder-diffraction data 
are lacking and the unit-cell data available. 

Advantages and Limitations 

The X-ray-diffraction method is advantageous for the analysis of un- 
known materials, especially mineralogic, because it reveals the state of 
chemical combination of the constituent elements. Furthermore, the meth- 
od is nondestructive, and the sample can be used for further studies by 
other methods. Moreover, satisfactory results can be obtained from very 
limited amounts of material. Small clusters of powder approximately 
0.1 to 0.2 mm. in diameter produce good patterns without necessitating 
objectionably long exposures. Under extreme conditions, suitable patterns 
have been obtained from samples that consisted of only a few micrograms 
of material. Likewise, only limited accuracy in measurements is necessary 
when making a qualitative analysis. Furthermore, a file of diffraction 
patterns constitutes a permanent record, which can always be examined 
and checked by anyone versed in the field. 

^ Hull., A, W., A New Method of X-ray Crystal Analysis: Phys. Rev., vol. 10, pp. 661 ff., 1917. 

'Hull, A. W., A New Method of Chemical Analysis: Am. Chem. Soc. Jour,, vol. 41, pp. 1168 £F., 
1919. 

* Clark, G. L., Applied X-Rays, New York, McGraw-Hill Book Co., Ino., 1940. 

' Davey, W. P., Study of Crystal Structure and Its Applications, New York, McGraw-Hill Book Co., 
Inc., 1934. 

'Barrett, C. S., Structure of Metals, New York, McGraw-Hill Book Co.., Inc., 1943. 

' Bunn, C. W., Chemical Crystallography (Interpretation of Data) , New York, Oxford Univ. Press, 
1946. 

' Hanawalt, J. D., Rinne, H. W., and Frevel, L. K., Ind. and Eng. Chemistry, Anal. Ed., vol. 10, 
pp 457 ff., 1938. 

' Card file index and first supplement compiled under the joint supervision of the American Society 
for Testing Materials and the American Society for X-ray and Electron Diffraction. These are available 
from the American Society for Testing Materials, 260 S. Broad Street, Philadelphia, Pennsylvania. 



216 



Subsurface Geologic Methods 



On the other hand, the X-ray-diffraction method is sometimes con- 
sidered rather limited as an analytic tool because the relative sensitivity 
requires that an appreciable amount of a constituent (from one to thirty 
per cent) ^^ must be present in a mixture before its presence can be 
detected. However, the use of improved techniques will do much to correct 
this situation. Some materials with patterns having reasonably low back- 
ground intensities and fairly strong lines can readily be detected in con- 
centrations as low as one-half to one percent, whereas other materials 
with weaker patterns, such as the montmorillonite-type clays, can be 




Figure 93. Schematic diagram of conventional powder camera. 



detected when present in amounts ranging from five to six percent of the 
sample. It has been reported that special treatment of this clay with 
glycerol permits detection in amounts as low as one percent. ^^ Limitation 
of the number of detectable constituents in mixtures due to crowding of 
lines has also been considered a disadvantage by some workers using 
small-diameter cameras with large pinhole systems. ^^ The use of larger 
camera (10 to 20 cm.) diameters, smaller pinhole systems, and longer 

■"' Brosky, S., P. T. L. Netos, Fittsburgh Testing Laboratories, Pittsburgh, Pennsylvania. 
'' Kelley, W. P., Cation Exchange in Soils: Am. Chem. Soc. Mon. 109, New York, Reinhold Publish- 
ing Corporation, 1948. 

^- Broskey, S., op. cit. , 



Subsurface Laboratory Methods 



217 



radiation wave lengths to spread out the patterns and increase the resolu- 
tion should increase the number considerably. The most important disad- 
vantages of the method are its inability to detect amorphous phases, such 
as glasses, when present in only limited amounts, and the fact that solid 
solutions may not always be observed. 

Apparatus for Recording the Diffraction Patterns 

Two general types of apparatus are commonly used for recording 
the X-ray-diffraction pattern. Both are essentially the same in regard to 
the generation of X-rays, being composed of a high-potentional (30,000 to 



Focal spot on 
torget of 
X-ray tube 



Focal spot on 
torget of 
X-ray tube 




Sample 



,, \ ^^ X Counter/ 
V \ "-^-^.slit- 




-Counter tube 



/ 1000- 1400 Volts 



y Poth of counter 
^ / tube 



Figure 94. (a) Schematic diagram of focusing powder camera, (b) Schematic dia- 
gram of relation of focusing camera to Geiger-Mueller-counter apparatus. 

50,000 volts) source of current, including a line voltage stabilizer, an 
auto-transformer to regulate the high potential, the necessary controls, 
rectifier, and X-ray tubes. The difference in these types of apparatus 
arises in the manner in which the X-ray-diffraction patterns are recorded; 
one type uses the conventional diffraction camera with photographic 
film and the other, a Geiger-Mueller-counter tube with a scaling circuit 
that may be used to measure the intensity of the diffracted rays, records 
intensity either by the counting technique or by automatic recording ap- 
paratus. The conventionl camera is shown schematically in figure 93. 



218 Subsurface Geologic Methods 

The Geiger-Mueller apparatus is constructed on the principle of the 
focusing powder camera (See a of fig. 94), although gross deviations 
from the principle have been permitted in the actual construction of the 
apparatus (See b of fig. 94). In the focusing camera the diffracted lines 
are focused so that they appear as sharp lines on the film, which lies on the 
circumference of the circle that passes through the slit. Inspection shows 
that even though gross deviations from the principles were made in the 
construction of the apparatus, it nevertheless has relatively good focusing 
at the countertube slit. On the other hand, even small irregularities in 
focusing will readily show up when line intensities are based on the num- 
ber of counts at the peak, and disagreement between the data of the two 
methods will result. The 29 angle, from which the diffraction effects are 
calculated by means of the Bragg equation, must be obtained by calcula- 
tion, using the geometric relations of the camera for patterns obtained with 
diffraction cameras employing films. For the Geiger-Mueller-counter appa- 
ratus the positions of the diffraction lines are obtained directly as the 26 
angle (in degrees) from a graduated arc on the countertube track. 

The film methods of recording the patterns were developed shortly 
after the interaction between matter and X-rays was discovered and have 
been improved gradually so that today their reliability is no longer ques- 
tioned. Furthermore, reliable methods of measuring and evaluating the 
data have been developed simultaneously, although they are not so short 
and simple as those used for Geiger-Mueller-counter data. Actually, film 
methods record and preserve a full, detailed pattern, giving such addi- 
tional information as orientation effects and particle sizes of every indi- 
vidual sample analyzed. All this information is lost in Geiger-Mueller- 
counter data. Furthermore, these effects, such as orientation, may cause 
appreciable error and distortions in the data without forewarning the 
operator. 

At present the reliability of some types of Geiger-Mueller-counter 
data is not well established, but further instrumental improvements and 
development and refinement of operational techniques may soon increase 
the reliability of the method. Variations in line intensity of twenty to 
thirty percent have been observed by workers ^^ studying platy minerals 
with Geiger-Mueller-counter apparatus. The effect of orientation on the 
intensity of the strongest line in the pattern has also been studied by the 
writer. Relatively soft, lathlike organic material was ground for 1| 
hours and the powder carefully packed into a special sample holder with 
a thin-bladed spatula. The sample was then made flush with the surface 
of the holder with a single pass of the spatula across the holder. The 
sample was approximately 2 cm. in diameter and 2^ mm. thick. The 
holder was so constructed that the sample could be rotated in steps about 
an axis perpendicular to the front surface of the sample. These steps 

" Beatty, Van IX, X-ray Spectrometer Study of Mica Powders: Am. Mineralogist, vol. 34, pp. 74 ff., 
1949. 



Subsurface Laboratory Methods 



219 



were taken at 15-degree intervals and several counts for each step were 
taken (to obtain an average value) with the counting technique through 
the peak of the strongest line in the pattern. The resulting intensity varia- 
tion is roughly shown in figure 95. The maximum variation observed for 
the strongest line of the pattern was found to be approximately 60 per- 
cent when the average mean intensity was used as a basis for the calcula- 
tion. Naturally, this procedure will give considerably more variation in 
intensity than will be observed among a group of samples that have all 
been prepared in essentially the same manner in the conventional type of 
sample holder. This difficulty of intensity variation might be overcome by 



100% 




90 



180 



270 



360 



Rotation of sample in degrees 

Figure 95. Variation of line intensity with rotation of powder sample obtained 
with a Geiger-counter instrument. 



220 Subsurface Geologic Methods 

adopting a sample holder capable of rotating the sample at a rate which 
would average out the orientation effects. 

The Geiger-Mueller-counter apparatus is especially recommended by 
the manufacturers for quantitative analytic work. The observations stated 
above, however, would introduce some question concerning this applica- 
tion. Furthermore, other research groups have found the apparatus unsuit- 
able for quantitative analytic work, especially when used with recorder 
apparatus.^* 

Conflicting statements concerning operating characteristics of Geiger- 
counter apparatus have also been made.^^ ^® 

It has been pointed out by Friedman ^^ that a microphotometer trace 
of a film pattern made with a conventional diffraction camera has so 
much intensity variation in the background that it is difl&cult to detect 
low-intensity lines in the pattern. However, inspection of the curve shown 
indicates that most of this variation was due to improper adjustment of 
the microphotometer on which the trace was made. Furthermore, this 
same paper also shows a comparable curve obtained with a Geiger-counter 
apparatus. The curve obtained with this apparatus has a uniform back- 
ground intensity as compared to the gradually diminishing (with larger 
Bragg angle) background intensity always obtained in microphotometer 
traces of film patterns made with diffraction cameras. The writer has ob- 
tained similar curves with a Geiger-counter apparatus furnished with a 
single defining slit producing a divergent X-ray beam. It was soon ob- 
served that at lower angles part of the X-ray beam spilled over at the ends 
of reasonably short samples commonly used, thus causing the uniform 
background (See b of fig. 94). On the other hand, a sample 16 cm. long 
gave a curve very similar in background intensity to microphotometer 
traces obtained from film patterns. Owing to the geometric shape of this 
sample and the setup of the apparatus, not all of the diffracted rays were 
gathered into the counter tube through the small slit directly in front of 
the counter tube. An even steeper background curve should have been ob- 
tained if the setup had been geometrically that of the focusing camera. 
Consequently, all lines recorded at lower angles must obviously have been 
abnormally weaker than they should have been if no part of the X-ray 
beam had spilled over at the ends of the sample. The amount of beam 
spillover was a function of the length of the sample and of the magnitude 
of the Bragg angle and was of considerable importance for the short sam- 
ples (2 to 3 cm. long) usually recommended. 



^* Klug, H. P., Alexander, L., and Kuramer, Elizabeth, Quantitative Analysis with the X-ray Spec- 
trometer: Anal. Chemistry, vol. 20, pp. 607 ff., 1948. 

^^ Carl, H. F., Quantitative Mineral Analysis with a Recording Diffraction Spectrometer: Am. Mineral- 
ogist, vol. 32, pp. 508 ff., 1947. 

^° Lonsdale, Kathleen, Note on Quantitative Analysis by X-ray Diffraction Methods : Am. Mineralogist, 
vol. 33, pp. 90 fif., 1948. 

■"Friedman, H., Geiger-Counter Spectrometer for Industrial Research: Electronics, vol. 18, pp. 132 
ff., 1945. 



Subsurface Laboratory Methods 221 

Preparation and Mounting of Specimen 

Too much emphasis cannot be placed on the selection of the sample 
for analysis. Because only a very small fraction of the sample placed 
in the camera is actually exposed to the X-ray beam, it is imperative that 
all precautions be observed in the choice and preparation of the sample. 
The sample chosen must be truly representative of the material being 
investigated as regards composition, structure, or other characteristics for 
which the sample is examined. 

Single Crystals 

Single-crystal patterns are seldom made when identification of the 
material is the only objective, as the powder method is usually consider- 
ably simpler. There may be occasions, however, when the sample is lim- 
ited to a very small, pure, single crystal, insufficient in amount to grind 
into powder. Under such circumstances, a single crystal ranging from 
0.5 mm. to several hundredths of a millimeter in cross section and from 
several millimeters to about 0.3 to 0.5 mm. in length is mounted on the 
end of a small glass rod or wire, with one crystallographic axis approxi- 
matelly parallel to the axis of the rod so that it can be mounted and ad- 
justed in the goniometer head of the single-crystal camera to turn about 
this axis. Patterns are recorded successively with alternate rotation about 
the three crystallographic axes according to procedures found in standard 
texts. ^^ From these patterns unit-cell calculations are made. Under adverse 
conditions it may be impossible to obtain patterns about all the crystallo- 
graphic axes, whereupon it may be necessary to calculate the dimensions 
of the entire cell from a single rotation pattern by means of the reciprocal 
lattice. A discussion of this concept is beyond the scope of this section, 
but it may be found in text books on X-ray-diffraction techniques. ^^ ^° ^^ 

Powders 

For powder patterns it is usually recommended that several milli- 
grams of representative material be crushed and ground in an agate (or 
mullite) mortar until the entire specimen will pass a 200-mesh silk bolting 
cloth or screen. The writer has observed that if the sample is turned or 
oscillated during exposure to the X-rays it will be sufficient to grind the 
sample until high lights from individual particles are no longer observed 
when the powder is examined in a bright light. If the material does not 
grind readily, it may be filed with a clean single-cut fine-tooth file, using 
no more pressure than is absolutely essential. If the specimen must be 
preserved in its original form, the specimen can be mounted in a suitable 
rotating or oscillating device in such a manner that the sample-to-film dis- 
tance remains constant. 



" Buerger, M. J., X-ray Crystallography, New York, John Wiley and Sons, Inc., 1942. 
" Clark, G. L., op. ci.. 
^ODavey, W. P., op. cit. 
2^Bunn, C. W., op. cit. 



222 Subsurface Geologic Methods 

Before the method of mounting the powdered specimen is selected, 
the optimum thickness of the sample to be used should be determined. 
The proper thickness can be calculated if sufficient information is avail- 
able concerning the specimen. Otherwise, the optimum thickness usually 
can be estimated approximately by an experienced operator from the 
amount of the undiffracted X-ray beam that penetrates trial specimens, 
as determined with a fluorescent screen. This thickness can be calculated 
from the equation :^^ 

2 
t=— 

where jx is the linear absorption coefficient calculated from the mass 
absorption coefficient according to the relationship: 






d being the density of the material, p the elemental fraction in the com- 
pound and ^ the mass absorption coefficients of the elements for the 

wave length of the radiation used. The values for ^ can be found in 

table form in volume 2 of "International Tabellen zur Bestimming von 
Kristallstrukturen," pages 577 and 578. 

For NaCl the optimum thickness for copper radiation is found to be 



'2;<7)= 



and 



dYp(~] =2.165 [.396X30.9+.604X103.4]=171 



t=^=m\l cm. 



This result indicates that the optimum sample thickness is usually con- 
siderably less than that generally recommended for capillary mounting.^^ 

If too thick a sample is used, a distorted pattern is obtained. Thus, 
it is obvious that sample thickness becomes important when deciding on 
a suitable mounting technique. Another very important factor to be con- 
sidered in connection with the mounting of the specimen is the amount 
of material available. 

For materials of high atomic weight the optimum thickness may be 
so small as to necessitate dilution of the crystalline material with amorph- 
ous diluents such as flour, cornstarch, or gum tragacanth.^* ^^ In any 

^ Buerger, M. J., op. cit., p. 182. 

*' Tentative Recommended Practice for Identification of Crystalline Materials by the Hanawalt X-ray 
Diffraction Method: Am. Soc. Testing Materials designation E43-42T, 1942. 

" Davey, W. P., op. cit. 

* Tentative Recommended Practice for Identification of Crystalline Materials by the Hanawalt X-ray 
Diffraction Method: Am. Soc. Testing Materials designation E43-42T, 1942. 



Subsurface Laboratory Methods 223 

case, however, these diluents should be avoided or kept to a minimum 
since some of them (e.g., raw cornstarch) produce a crystalline pattern 
of their own, or an amorphous pattern with very broad lines (halos) . 
These superposed patterns of the diluents often cause a considerably 
localized background fog, with consequent difl&culty in observing lines in 
the regions of the amorphous bands. 

It is recommended that the ground and diluted samples be packed 
into capillary tubes with an inside diameter of 0.4 to 0.6 mm. and made 
of plastic materials (materials with amorphous patterns) or glass con- 
taining elements of only low atomic weight. The plastic materials are 
preferred to glass, as measurements on Pyrex tubes with wall thickness 
just sufi&cient to permit careful handling show forty- to fifty-percent 
absorption of the CuKa radiation. Longer wave lengths are absorbed to 
an even greater extent. Glass appears to be suitable for MoKa radiation; 
however, as will be shown later, Mo radiation is not desirable for use in 
the identification of components of mixtures. 

Another mounting method recommended for long-wave-length studies 
on materials of low atomic weight consists in mixing the powder of the 
unknown with about ten percent (by volume) of gum of tragacanth or 
collodion and extruding it as a rod approximately 0.5 mm. in diameter. 

An excellent method of mineral specimen preparation used by some 
of the most prominent workers in the field, although it is usually not de- 
scribed in standard texts nor recommended in the American Society for 
Testing Materials procedures,^'' consists in mixing the powder of the 
unknown with a minimum of Dupont household Duco cement (or other 
plastic cements) and then rolling the plastic mass between two microscope 
slides to form a thin rod of the desired thickness. Thickness can be care- 
fully controlled by inserting the microscope slides in a jig which holds 
them a fixed, predetermined distance apart. The cement acts as binder 
and diluent, and if kept to a minimum generally will not affect the back- 
ground of the diffraction pattern. The writer has found this method to be 
particularly desirable for identifying montmorillonite-type clays, as the 
Duco cement conditions the clay so that it needs not be specifically 
treated "^ ^^ ^^ to be differentiated from other materials, such as muscovite 
or illite. 

Platy or fibrous crystals may become oriented in the cement during 
rolling of the rod. The lack of random orientation changes the circular 
lines of the pattern to arcs, especially the lines formed at a small angle 
to the beam. On film patterns orientation effects usually do not cause 
any difiiculty where qualitative identification is the objective, so long as 

^' Tentative Recommended Practice jot Identification of Crystalline Materials by the Hanawalt X-ray 
Diffraction Method: Am. Soc. Testing Materials designation E43-42T, 1942. 

" Jackson, M. L., and Hellman, N. N., X-ray Diffraction Procedure for Positive Differentiation of 
Montmorillonite from Hydrous Mica: Soil Sci. Soc. Am. Proc, vol. 6, pp. 133 £f., 1941. 

' Hellman, N. N., Aldrich, D. G., and Jackson, M. L., Further Note on an X-ray Diffraction Pro- 
cedure for the Positive Differentiation of Montmorillonite from Hydrous Mica: Soil Sci. Soc. Am Proc. 
''ol. 7, p. 194, 1942. 

Bradley, W. F., Diagnostic Criteria for Clay Minerals: Am. Mineralogist, vol. 30, pp. 704 £f., 1946. 



224 SUBSUKFACE GEOLOGIC METHODS 

the film is wide enough to include all orientation arcs. On the other 
hand, these effects frequently are advantageous in that they give some 
idea concerning the orientation of the crystallographic planes producing 
these arcs. Moreover, because the orientation arcs are darker than would 
be tlie equivalent complete circular line, lower percentages of platy or 
fibrous minerals can be detected in mixtures than would otherwise be 
detected. 

Furthermore, use of the Duco cement permits preparation of a 
thinner sample than would the recommended capillaries and consequently 
makes it possible to obtain a pattern with narrow, sharp lines with maxi- 
mum resolution. With this type of sample mounting, several lines are 
frequently obtained at the average position of a single broad line reported 
in the literature. 

Other methods, such as affixing the powder to strings, hair, wire, and 
glass rods, have been suggested also.^*' These mounts commonly produce 
abnormal effects and do not appear desirable because of the difficulties 
involved in obtaining representative samples and the large amount of 
foreign material (the rod and binder) included in the sample. With 
glass rods, double lines frequently are obtained in the pattern, a condi- 
tion that is very undesirable, especially in analysis of mixtures. 

If only a very limited amount of material is available, a small lump 
0.1 to 0.2 mm. in diameter can be mounted with mucilage or Duco 
cement on the end of a very thin glass rod. For very fine-grained mate- 
rials in which the particles are randomly distributed, a powder diffraction 
is obtained. If the particles are not arranged randomly, the materials 
first should be crushed with a miscrospatula and the powder worked into 
a tiny ball with a binder. Such samples require no more than a few 
micrograms of material and produce satisfactory patterns at approximately 
double the usual exposure time. 

Recently a camera has been developed in the Bureau of Reclamation 
laboratories to study materials in petrographic thin sections that are not 
identifiable by microscopic methods. The area on which the pattern 
is obtained is approximately 0.008 inch in diameter, or about twice the 
thickness of an average sheet of paper. For this procedure the slide is 
warmed to soften the mounting medium, and the thin section slid over so 
that the region to be studied projects over the edge of the glass slide. The 
cover glass is retracted at the same time. The specimen is mounted in the 
camera under the petrographic microscope to insure centering of the 
selected area in the beam. The sample is rotated during the exposure to 
produce smooth, uniform lines in the pattern. After the pattern has been 
recorded, the slide is again warmed, and the thin section returned to its 
original position and covered with the original cover glass. 

At present, reliable methods for mounting powder samples for studies 



^^ Tentative Recommended Practice for Identification of Crystalline Materials by the Hanawalt X-ray 
Diffraction Method: Am. Soc. Testing Materials designation E43-42T, 19-12. 



Subsurface Laboratory Methods 225 

with the Geiger-counter apparatus seem to be lacking. H. F. Carl has 
described a method which he found to yield satisfactory quantitative accu- 
racy.^^ Whatever method is selected, it should be remembered that differ- 
ent materials pack differently into the holder, and the operator should 
first check his technique on a series of synthetic samples of known compo- 
sition before attempting to use it quantitatively or on unknown specimens. 

Position and Type of Film 

As indicated in figure 92, diffraction lines (rings) can be produced 
over the entire region from practically 0° to 175° of the 29 angle. Some 
materials such as metals and inorganic compounds produce patterns rang- 
ing over the entire region from 0° to 175°, whereas organic compounds 
produce practically an entire pattern at small angles. Again, certain sec- 
tions of the region from 0° to 175° may be selected for detailed study, 
as for example in back-reflection work or studies where extreme accuracy 
is involved, when the region from 130° to 175° is used (See fig. 96). Con- 
sequently, the type of camera and film selected depends upon the objec- 
tive of the investigation. Thus, for example, a camera with the film in the 
form of a cylinder with the specimen located at the axis is to be much 
preferred for the identification of rocks, minerals, and soils. 

Most, if not all. X-ray-film emulsions available were developed pri- 
marily for radiographic work and consequently have a rather high degree 
of contrast ^" or reveal relatively small differences in absorption by the 
materials studied. For diffraction work, especially for studying mixtures, 
a film showing a straight-line function with a moderate slope over a con- 
siderable range, when the line density, log f -^ j , is plotted against expos- 
ure, {lot), is desirable (See fig. 97), At present, such film is not generally 
available, and one must use the emulsions developed for radiographic 
work, compromising between exposure time and pattern quality. The 
fastest emulsions usually show considerable background blackening, where- 
as slower films producing good, clean backgrounds require considerably 
more exposure. Thus the choice of film rests on a number of conditions. 
For rapid and only approximate identifications, the fast films are pre- 
ferred, whereas slower films are used if all possible information is to be 
gleaned from the pattern. Films as a rule are duplitized; that is, they 
have emulsions on both sides. To a slight extent, the double emulsion 
causes diffuseness in the lines, but rarely sufficiently to justify use of single- 
layer-emulsion film. All films should be developed according to the time, 
temperature, and processing conditions recommended by the manufac- 
turer.^^ 

Intensifying screens have been used for cutting down exposure time, 
but this practice is not recommended for mixtures of minerals because the 

" Carl, H. F., op. cil. 

Radiography of Materials, Rochester, N. Y., Eastman Kodak Co., X-ray Division. 
Radiography of Materials, Rochester, N. Y., Eastman Kodak Co., X-ray Division. 



226 



Subsurface Geologic Methods 



screens broaden the pattern lines and thereby decrease definition. Patterns 
so produced have little value for determining the constitution of complex 
mixtures, for maximum definition with minimum line width is desired to 
permit measurement of the maximum number of lines. 

X-Radiation 
The use of the Ka doublet radiation from molybdenum has been rec- 
ommended for chemical analysis by the X-ray-diffraction or Hanawalt 
method.^"* This radiation might be suitable for the identification of pure 




Back reflection 
film 



Cylindricol film-' ^-Flof film 

Figure 96. Film positions in various cameras used for powder studies. 



TABLE 8 

Angular Range of Corresponding Patterns Produced by Common Target 
Materials Compared to Pattern Range of Chromium Ka Radiation 



Radiation 


Angular range of corresponding patterns 
(for outer line of pattern with d = 1.1497 A.) 


Cr 

Fe 

Co 

Cu 

Mo 


170° 0' 

114° 48' 

102° 10' 

84° 18' 

36° 0' 



^* Tentative Recommended Practice for Identification, of Crystalline Materials by the Hanawalt X-ray 
Diffraction Method: Am. Soc. Testing Materials designation E43-42T, 1942. 



Subsurface Laboratory Methods 



227 



substances or very simple mixtures, but does not appear to be of much 
value for complex mixtures such as rocks and soils, for which the patterns 
should be spread out as much as possible to prevent superposition of 
lines from the different patterns of the constituents in the mixture. Refer- 
ence to table 8, which shows the angular range of corresponding patterns 




Exposure (intensity x time) 
Figure 97. Exposure-density curve for typical X-ray film. 



produced by the common target materials available as compared to the 
full pattern range (170°) for chromium Ka radiation, should remove any 
doubt concerning the foregoing statement. The results given in this table 
should enable the operator to choose the radiation for his particular needs. 
Where the operator is restricted to a single type of radiation, the Ka of 



228 Subsurface Geologic Methods 

copper is chosen almost invariably because it favorably combines sample 
penetration with a reasonably expanded pattern of good quality. 

Since the K X-ray spectrum always contains characteristic radiation 
of several wave lengths, suitable filters ^^ ^^ or a crystal monochromator 
should be employed to produce reasonably monochromatic radiation and 
thus avoid superposition of lines from a second pattern derived from Kp 
radiation. In patterns of pure substances or very simple mixtures, the 
position of Kp lines can be calculated and the lines disregarded in the 
interpretation of the data. However, the Kp radiation should be removed 
when making patterns of mixtures, as such patterns are always very 
complex and the presence of K^ lines serves only to cause errors and 
confusion. 

Measurement of Lines in Pattern and Conversion to d Values 

The X-ray pattern usually must be measured and the data used to 
determine interplanar spacings or unit-cell dimensions. Consequently, all 
precautions must be taken in the processing of the film to avoid film 
shrinkage or reduce it to negligible amounts. Film shrinkage will be 
negligible if the camera is calibrated against a pattern obtained from a 
known substance such as NaCl, the film of the standard pattern having 
been developed according to a standard procedure, which thereafter is 
followed explicitly in the development of all patterns obtained with that 
camera. Film shrinkage has been found to increase with washing time, 
especially if prolonged and the shrinkage is not uniform throughout the 
entire film.^^ Developing procedures can be checked for film shrinkage 
by exposing or marking on the film fixed lengths before processing. Cor- 
rection for shrinkage is also frequently made through the use of an internal 
standard such as NaCl, the line positions of which are accurately known, 
the pattern for NaCl being superposed directly on the pattern of the 
unknown. 

A number of measuring devices are offered by manufacturers of 
X-ray apparatus with which either the diameters or radii of the powder 
rings can be rapidly and accurately measured in units of length, usually 
centimeters. These devices could be calibrated directly in KX or A, units 
(A. = 1.00202 KX units) , but calibration in this way restricts their use to 
a single type of camera with a fixed radius. Consequently, the measure- 
ments in centimeters must be converted into interplanar spacings or unit- 
cell dimensions by calculation or calibration curves indicating ring diame- 
ters or radii (in centimeters or millimeters) as a function of interplanar 
spacing (in angstrom units) . For rapid and approximate measurements, 
a direct-reading scale on transparent plastic can be prepared from calcu- 
lated or plotted data, interplanar distances equivalent to each ring being 
read directly when the scale is superimposed on the pattern. 

^ Clark, G. L., op. cit. 
'^Bunn, C. W., op. cit. 

" Claassen, H. H., and Bow, K. E., Correction of X-ray Powder Diffraction Patterns: Sci. Inst. Rev., 
vol. 17, pp. 307 £f., 1946. 



Subsurface Geologic Methods 



KX units 



EstiFJited 

relative 

intensity 



Order 

Ox 

intensity 




Plate 6. 



illustration of tiie use of tiie card-index method of 
identifying an unknown. 



Subsurface Laboratory Methods 



229 



If single-crystal-rotation patterns taken perpendicular to each of the 
three crystallographic axes are available, one dimension of the unit cell 
can be calculated from each of the three patterns. For cylindrical pat- 
terns the angle «„ is calculated from the tangent function of the distance 
measured on the pattern between the and nth layer line and the film ra- 
dius. For flat patterns it is calculated from the distance measured between 
the layer and the apex of the nth-layer-line hyperbola and the sample-to- 




Cylindrical film 



Flat film 



Figure 98. Schematic diagram of single-crystal layer-line positions in cylindrical- 

and flat-film cameras. 



film distance (See fig. 98). This value is then substituted in the equation: 



/= 



nX. 



sin Un 



to obtain the identity period, /, or the distance between planes from one 
equivalent point to the next along the axis of rotation.^^ 

On the other hand, if the diffraction data were obtained by the powder 
method, a technique considerably simpler than the single-crystal rotation 
method, the procedures of measurement and calculation are different. 



'* Friedmaii, H., op. cit., chap. 5. 



230 Subsurface Geologic Methods 

The process of measuring the powder pattern is the same regardless of 
how the measurements are to be used. The diameters (or radii) of all 
lines in the pattern are measured and recorded in centimeters or milli- 
meters. If the X-ray pattern is recorded on flat film, the Bragg angle is 
obtained from the tangent relationship, namely, 

line radius 
tan 26=- 



sample to film distance 



With the Bragg angle 6 known, the interplanar distances, d, can then be 
readily calculated by means of Bragg's law. 



a=- 



2 sin 9 



With the powder pattern recorded on cylindrical film the Bragg angle in 
degrees is obtained from the relationship, 



, „ ^360 90a 

2^=(-iTlTr- or e = —^ 



_/a\360 
~\r) 2tt 



where a is the line radius on the film in millimeters and R is the film 
radius (camera radius) in millimeters. Substituting this value of 6 in 
Bragg's law we obtain ^^ 

nX 
d = 



o . /90a\ 



In addition to this calculation, the relative line intensities must be 
evaluated. The intensities are expressed in some system, for example 
as V.V.S. to V.V.W. (very, very strong through various gradations to 
very, very weak), or on a numerical basis ranging from 10 to 1 or 1 to 
0.01. Visual approximation of intensity is sufficient, as a trained operator 
can see all the details in a pattern that can be detected with a densi- 
tometer. As has been suggested previously (See fig. 97), relative intensi- 
ties of lines in a well-exposed pattern are diff'erent from those in an 
underexposed pattern. Differences in exposures not only result from 
changes in exposure time from specimen to specimen, but also occur within 
a single pattern representing a mixture containing both large and small 
proportions of the several ingredients.^'^ If a particular constituent is to 
be determined, the writer has found it advisable to prepare a series of 
underexposed patterns of the constituent in pure form with exposure 
times of 1 percent, 2.5 percent, 5 percent, etc., of that used in obtaining 
the pattern of the mixture. This procedure will demonstrate why rather 



^ Clark, G. L., op. cit., p. 279. 

'"' Hellman, N. N., and Jackson, M, L., Photometric Interpretation of X-ray Diffraction Patterns for 
Quantitative Estimation of Minerals in Clays: Soil Sci. Soc. Am, Proc, vol. 8, pp. 135 ff., 1944. 



Subsurface Laboratory Methods 231 

strong lines of the patterns of minor constituents frequently cannot be 
found in the pattern of the mixture. 

Identification of Minerals and Components of Mixtures 

Single Crystals 

If X-ray-diffraction data have been obtained from single-crystal rota- 
tion patterns and unit-cell dimensions calculated, the identity of the com- 
pound usually can be determined. However, the determination may not 
be simple because as yet unit-cell data have not been compiled into tables 
according to some regular order (decreasing or increasing) of the cell 
dimensions along the three axes. However, if the unit-cell dimensions 
of the unknown are found to correspond to those of a previously de- 
scribed compound, the identification can be considered to be reliably 
established. In fact, unit-cell data are about the most reliable type of 
X-ray data available for identifying organic materials; and it is most 
regrettable that no one has undertaken the task of compiling them into 
some systematic form based upon dimensions. Recently R. W. G. Wyckoff ^^ 
initiated a continuous loose-leaf system for compiling unit-cell and other 
crystallographic data according to compound classification. This compila- 
tion will aid in the identification of compounds, but the diflficulties of 
accomplishing an identification from unit-cell dimensions alone will be 
manifest. 

Powders and Fine-Grained Materials 

If the X-ray-diffraction data were obtained from powder patterns, 
the process of qualitative and semiquantitative identification is consider- 
ably simpler than if only single-crystal rotation patterns were available. 
For identification of a specimen from a powder-diffraction pattern, the 
radii, or diameters, of all lines in the pattern are measured, and the inter- 
planar spacings calculated. The details of the procedure to be followed 
depend on the nature of the unknown and on the amount of other data 
available, such as optical and physical properties and chemical analyses. 

If the unknown represents a pure compound or a mixture composed 
essentially of one constituent with only minor amounts of other ingredi- 
ents and nothing is known concerning the identity of the compound or 
the principal ingredient, the Hanawalt method of identification is used. 

The Hanawalt method, recommended by the American Society for 
Testing Materials, is based upon a card-file index system catalogued 
according to the three strongest lines in the pattern. After the pattern 
of the unknown has been measured, converted into interplanar spacings, 
the intensity of the lines estimated, and at least the three strongest lines 
(more if the three strongest lines are not outstanding) identified, the group 
of cards representing materials for which the strongest line corresponds 
to the same interplanar spacing as does the strongest line in the pattern 

^^ Wyckofif, R. W. Gv, Crystal Structures, New York, Interscience Publishers, Inc., 1948. 



232 Subsurface Geologic Methods 

is selected from the index. The subgroup for which the second-most- 
intense line corresponds to the same interplanar spacing as does the 
second-strongest line in the pattern of the unknown is then examined for 
correspondence between the third line of the cards and the third-strongest 
line in the pattern. Finally, the entire pattern of the unknown is checked 
against the pattern selected from the card index. This procedure is illus- 
trated in plate 6. However, because of differences between the techniques 
used in obtaining the data for the card index and that used by the oper- 
ator in obtaining the pattern of the unknown, or because of variations 
found in the patterns of some types of materials (to be discussed later) , 
the operator should regard correspondence within ±: 0.05 A. as a satis- 
factory match for interplanar spacing in comparing his patterns with 
those recorded in the index. This same possible variation should be 
allowed in selecting the groups of cards for comparison. 

Should the foregoing procedure be unsuccessful or if the specimen 
to be identified is known to be a mixture of several ingredients all in 
only small or moderate concentration, a somewhat different method of 
identification must be used. In mixtures, each of the three strongest lines 
may belong to patterns of different constituents so that the above procedure 
(outlined in plate 6) could not be used. For relatively simple mixtures, 
the procedure above may work if more (ten or twelve) of the strongest 
lines are used in searching the card index. In general, however, only the 
strongest line of the pattern can be used as a guide for selecting the 
group of cards for comparison. All of the lines on each card of the 
selected group are compared with the pattern of the unknown; bearing 
in mind, of course, that at least all the strongest lines must be found in 
the pattern of the unknown, with proper relative intensity. Checking of 
only a few lines on a card is usually sufficient to indicate whether or not 
agreement exists. When a card identifies part of the pattern of the mix- 
ture, the lines belonging to the pattern of the identified constituent are 
marked (on the pattern or a corresponding tabulation of data). The 
procedure is now repeated for the remainder of the pattern, again start- 
ing with the strongest remaining line. In this way all the constituents 
of the mixture can be identified, provided their patterns are catalogued 
in the index, when fluorescent scattering is small (recognized by light back- 
ground in the X-ray pattern). The relative amounts of the ingredients 
present are deduced from the relative intensities of the lines in the pattern, 
as compared to the intensities of the lines in the pattern of the pure con- 
stituents, the exposure times, of course, being the same for all patterns. A 
series of underexposed patterns (1, 2^, 5, etc. percent of the total exposure 
time) of the pure constituent in question will be of considerable help in 
estimating these intensities. 

If the absolute proportion of each compound in the mixture is to 
be determined, a synthetic specimen must be prepared from the identified 
pure materials in such proportions that the synthetic mixture yields a 



Subsurface Laboratory Methods 233 

pattern matching in spacing and intensity all the lines of the original 
pattern, when both patterns are prepared under identical conditions of 
exposure and processing. If line shifts, fading of the pattern in general 
with increasing values of the 29 angle, or other differences are observed 
in the patterns, irregularities of composition, such as solid solutions, are 
indicated and the compound composition of the specimen must be deter- 
mined by calculation from a chemical analysis. The chemical analysis 
frequently is best accomplished by means of standard spectrographic pro- 
cedures. For thorough study of mixtures of silicates, the methods of 
X-ray-diffraction analysis *^ ^^ ** are practically indispensable. These meth- 
ods reveal the various chemical combinations in which the silicon exists, 
whereas chemical or spectrographic methods alone yield only the total 
amount of silicon in the unknown, giving no clue as to its mode of com- 
bination. 

Recently the American Society for Testing Materials has announced 
the completion and availability in the near future of the new second 
supplementary set of index cards. The original and first supplementary 
sets will now be available in the revised form only. Each set includes data 
for approximately 1,400 compounds. 

In the original and first supplementary sets, the values of the d-spac- 
ings corresponding to the three strongest lines, together with their 
corresponding relative intensities, appear in the upper left-hand corner of 
each card (See pi. 6). There are three cards in the file for each diffrac- 
tion pattern; the first card has the strongest line of the pattern at the 
extreme left and also contains the complete pattern data and some crystal- 
lographic data where available. The second card has the second strongest 
line in this position, and the third has the third strongest line in this 
position. The cards with the second and third strongest lines at the extreme 
left position were only "follow" cards and did not contain any data other 
than the d-spacings corresponding to the three strongest lines. The cards 
are filed in straight numerical order. 

The revised original and first supplementary sets and the second 
supplementary set include only one card for each pattern, so as to reduce 
the required number of cards. These cards also include the data for the 
d-spacings corresponding to the three strongest lines of the pattern listed 
in decreasing order of intensity in the upper left corner of the card. The 
data for the largest spacing of the pattern are given to the right of the 
data for the three strongest lines. Wherever available, additional data 
consisting of the data for the X-ray set-up, crystallographic information, 
optical information, and information concerning the source, preparation, 
heat treatment, etc., of the sample are given. In addition, the card con- 

^ Clark, G. L., and Reynolds, D. H., Quantitative Analysis of Mine Dusts: Ind. and Eng. Chemistry 
Anal. Ed., vol. 8, pp. 35 ff., 1936. 

*^ Ballard, J. W., Oshry, H. I., and Schrenk, H. H., Quantitative Analysis by X-ray Diffraction I. 
Determination of Quartz: Bur. Mines Rept. Inv. 3520. 

■•^ Ballard, J. W., and Schrenk, H» H., Routine Quantitative Analysis by X-ray Diffraction: U. S. 
Bur. Mines Rept. 3888. 



234 Subsurface Geologic Methods 

tains the formulas (chemical and structural for organic compounds), 
name, and complete pattern data. The cards are arranged into small 
Hanawalt groups of convenient size for values of the strongest line, and 
each group is arranged in numerical sequence according to the values of 
the second strongest line. This difference between the old and the revised- 
card indices will, of course, alter the above-described procedure somewhat 
when the revised index is used. With the revised index, the search of the 
diffraction-data file starts with two lines chosen from the unknown pattern 
as the strongest and second strongest. If this choice does not locate a 
corresponding X-ray pattern, it is necessary to reverse the order of the 
lines and search again. 

It may even be necessary to try various other combinations of strong 
lines in the pattern before the identification can be made. For those who 
wish to continue the original method of searching the data file, the Society 
offers additional sets of the revised cards at reduced prices. A numerical 
index is also supplied with the revised sets of cards. This index has listings 
arranged in Hanawalt groups with three variations for the three strongest 
lines in each pattern: namely, first, second, third; second, first, third; and 
third, first, second. 

When considerable investigation is being carried out in a limited 
field, or if sufficient optical or other data are available so that the possible 
compounds in unidentified specimens are relatively small, it frequently 
is advantageous to build up a file of patterns of standard materials. These 
patterns can then be used for identifying unknowns by direct comparison 
with their patterns. Plate 7 illustrates this method. However, it is to be 
strongly emphasized that extreme caution must be observed in selecting the 
materials for these standard patterns. Errors in identification are found 
frequently even for specimens obtained from established museum and 
private mineral collections. 

Direct comparison of patterns, when used together with the Hanawalt 
method described above, is the most satsifactory for identification of 
materials, both in accuracy and time saved in the analysis. Occasionally, 
the Hanawalt method fails for mixtures because several strong lines of 
different ingredients fall in juxtaposition on the pattern and consequently 
are considered as a single broad line in the interpretation of the data, 
thus considerably displacing the position of the line in question. If the 
probable constitution of the mixture can be surmised, direct comparison 
with standard patterns will immediately disclose such situations, and errors 
and time-consuming labor are avoided. 

Sometimes unit-cell data are available in the literature when powder 
data are lacking. Unit-cell data for a known material can be used to 
establish the identity of an unknown material from which a powder-dif- 
fraction pattern has been obtained. This method is practicable only if 
some clue suggests the identity of the unknown, and the number of known 
materials to be compared with the unknown is small. The comparison of 




o 

a 

s 



O 




Subsurface Laboratory Methods 235 

the unit-cell data with the powder-diffraction data is accomplished by 
application of the reciprocal-lattice concept. A complete explanation of 
this concept is, of course, beyond the scope of this section and the reader 
is referred to other sources.*^ *^ ^^ However, it can be shown that the re- 
lationship between the true lattice (real space) and the reciprocal lattice 
(reciprocal space) can be expressed by the equation, 

, RX 
d = , 

d* ' 

where d is the interplanar distance in the true lattice, c?* the interplanar 
distance in the reciprocal lattice, A the wave length of the radiation used, 
and R a constant called the "magnification factor" applied to convert 
the dimensions in reciprocal space to such a magnitude that the reciprocal 
lattice or net can be plotted easily in cm.-units. If the unit-cell dimensions 
are not much over 10 A., the value 7? = 10 will produce a reciprocal net 
of convenient dimensions. If the unit cell has dimensions between 10 and 
30 A., a value of 7? = 20 should be chosen. Briefly, the procedure is the 
following: the a, b, and c dimensions of the unit cell are converted into 
reciprocal-cell dimensions by means of the equation above and the result- 
ing three-dimensional net plotted in one plane by folding the vertical planes 
down into the horizontal plane (See fig. 99). Thereupon, the experiment- 
ally determined powder-diffraction data are also converted into reciprocal 
dimensions by the same equation and the results (rings representing the 
ends of reciprocal space vectors free to turn about the origin) are super- 
posed on the reciprocal net of the unit cell. If the unit cell fits' the experi- 
mentally determined powder-diffraction data, there will be a net intersection 
at the end of each vector ; i.e., the rings derived from the powder data will 
all pass through one or more intersections of the three-dimensional recip- 
rocal-unit cell net. 

A mixture of minerals which are frequently difficult to differentiate 
by optical examination, especially when examined in the form of a rather 
fine powder, has been chosen to illustrate this method. Owing to the nature 
of these minerals, the mixture could be identified as a single homogeneous 
substance. A diffraction-powder pattern, however, will definitely show it 
to be a mixture. With the methods described above, one constituent can 
readily be identified as quartz from these powder data. This identification 
is further verified by direct comparison with a standard quartz pattern 
(See pi. 7). 

Assuming that powder data are not available for the other constituent 
of the mixture, it would then be impossible to identify this constituent 
with the aid of the card index. However, if now through more thorough 
optical examination, further data can be obtained to limit the number of 
possible compounds to be checked to a reasonable number, identification 

** Clark, G. L., op. cit. 
*«Davey, W. P., op. cit. 
« Bunn, C W., op. cit. 



236 



Subsurface Geologic Methods 



will still be possible if suitable unit-cell data are available. In this case, 
this would involve careful checking of the refractive indices, obtaining the 
birefringence, and, if possible, such information as would enable one to 
classify the constituent as isotropic, uniaxial, or biaxial. Now further 
checking of the list of possible constituents obtained by the above pro- 
cedure against the list for which powder data are available would readily 




Figure 99. Comparison of powder data of unknown with possible 
unit-cell data. 



reduce the possibilities so that it would become feasible to apply the 
reciprocal-lattice method shown in figure 99. 

All unidentified lines of the pattern of the unknown are converted 
to lines with reciprocal radii by means of the above equation and then 
drawn on transparent paper or plastic. The unit-cell dimensions of the 
possible constituents are then converted to reciprocal dimensions, and these 



Subsurface Geologic Methods 




Plate 8. Typical clay patterns. (1) "Wyoming bentonite (montmorillonite) , (2) 
beidellite, (3) hectorite, (4) nontronite, (5) glauconite, (6) kaolinite, (7) 
dickite, (8) illite. 



Subsurface Laboratory Methods 237 

reciprocal three-dimensional nets are drawn on separate sheets of paper. 
For figure 99 the unit-cell dimensions are those of cordierite: namely, a = 
17.1 A., b = 9.78 A., c = 9.33 A., and the orthorhombic crystal system. 
The experimental data are then superimposed on these various possible 
nets and the experimental lines (rings) checked for agreement with the 
net intersections. If a reasonable number of lines show agreement, then 
any lines not identified by the net intersections directly as (hOO), (kOO), 
(100), (hkO), (hOl) and (Okl) are checked for (hkl) agreement (dotted 
triangles in figure 99 represent coincidence of (hkl) net intersections 
with experimental data lines) . Coincidence of one or more net inter- 
sections with every experimental powder-data line identifies the second 
constituent in the mixture as cordierite. 

Special Problems of Identification 

The value of the X-ray-diffraction method, especially as a research 
tool, cannot be questioned. Its greatest effectiveness is derived when the 
X-ray-diffraction data are supplemented by physical and chemical deter- 
minations made by other methods; but the method can be used inde- 
pendently to great advantage in many problems. In some investigations 
X-ray-diffraction analyses are more rapid and efficient than are alterna- 
tive methods; in other investigations, X-ray-diffraction analysis only will 
yield the necessary information. Jn plates 8 and 9 are examples of ma- 
terials that are difficult to analyze or identify by methods other than X-ray- 
diffraction analysis. 

Among the clays (pi. 8) some members of the montmorillonite 
group ^^ ^^ show remarkable similarity, and at present information is 
insufficient to permit positive differentiation of the several members of 
the group on the basis of the X-ray-diffraction patterns alone. However, 
if the clays are calcined at temperatures determined from experimental 
studies or from thermal-dehydration ^° or differential-thermal ^^ ^^ analyses, 
their identity can be established definitely from X-ray-diffraction studies. 
Furthermore, when Dupont household Duco cement is used as a binder 
for the powdered montmorillonite-type clay samples, the diameter and 
sharpness of the innermost line in the pattern give some information con- 
cerning the identity of the adsorbed cations. Preliminary observations in- 
dicate that a broad diffuse line represents a mixture of cations, whereas a 
sharp narrow line represents a relatively pure single cation. For the 
potassium ion, the line position corresponds to approximately 11.9 A., for 
the sodium ion approximately 12.9 A., and for the calcium ion approxi- 
mately 15.5 A. Likewise, the general degree of expansion or contraction 



^3 Grim, R. E., Modem Concepts of Clay Minerals: Jour. Geology, vol. 50, no. 3, pp. 225 S., 1942i. 

■** Ross, C. S., and Hendricks, S. B., Minerals of the Montmorillonite Group: U. S. Geol. Survey 
Prof. Paper 205-B, 1943 

^^ Nutting, P. G., Some Standard Thermal Dehydration Curves of Minerals: U. S. Dept. Interior Prof. 
Paper, 197-E. 

^^ Grim, R. E., and Rowland, R. A., Differential Thermal Analysis of Clay Minerals and Other 
Hydrous Minerals: Am. Mineralogist, voi. 27, pp. 746, 801 ff., 1942. 

^^ Grim, R. E., Differential Thermal Curves of Prepared Mixtures of Clay Minerals: Am. Mineralogist, 
vol. 32, pp. 493 ff., 1947. 



238 Subsurface Geologic Methods 

of the over-all pattern is an indication of the chemical nature of the 
middle, gibbsite or brucite, sheet of the three-layer packet. For an element 
with a small atomic radius (aluminum) an expanded pattern is obtained, 
whereas for an element with a large radius (ferrous iron) a contracted 
pattern results. In general, most clays can readily be recognized and 
identified without preliminary treatment, as is shown by plate 8. The 
X-ray-diffraction method is particularly valuable in the analysis of shales, 
because they are frequently so fine-grained and so heterogeneous in com- 
position as to preclude adequate microscopic analysis. The optimum par- 
ticle size for X-ray-diffraction studies ranges from about 10"^ cm. to 10'^ 
cm., which lies just beyond the limit of the microscope. Plate 9 illustrates 
differences between various shales. 

However, the X-ray-diffraction method is not a panacea for all prob- 
lems, and its utility is usually considerably enhanced if it is used in con- 
nection with other methods, especially microscopic, spectrographic, and 
chemical procedures. This is particularly true for investigation of certain 
types of complex minerals or mixtures. There is no doubt that the method 
becomes more effective and efficient as the mixture becomes simpler or 
the unknown material purer; and, consequently, it is at times advisable 
or even necessary to concentrate or purify the constituents for separate 
study before a mixture can be satisfactorily analyzed. Purification and 
concentration of ingredients are especially valuable in the investigation 
of substances whose pattern is not sufficiently distinctive to permit use of 
merely a few isolated lines. Optical data obtained from microscopic 
measurements can often reduce time and labor by aiding in the selection 
of standard patterns to be used in the comparison; of course, positive 
identification may be accomplished on some materials by microscopy alone. 
Much time can be saved in a laboratory by using the X-ray method only 
if satisfactory answers cannot be obtained from microscopic studies. 

The fact that nature is not particular, as regards chemical composi- 
tion, when forming crystals is being recognized by men of science.^^ ^* ^^ 
Very important properties are associated with apparently insignificant 
changes in chemical composition of minerals. Frequently, when once a 
geometric space arrangement of a crystal has been started, nature will 
continue with the building process using indiscriminately any atoms or 
ions available that are reasonably similar in size as long as the over-all 
structure is kept electrically neutral. Crystals that have extensive sub- 
stitution have been referred to as "half-breed" and "stuffed" crystals,^® 
depending on the mechanism by which the structure maintains neutrality. 
As a result of such partial substitutions the refractive indices of some 



" Thompson, J. B., Jr., Role of Aluminum in Rock-Forming Silicates: Am. Mineralogist, vol. 33, 
pp. 209 ff., 1948. 

"^ Buerger, M. J., Crystals Based on the Silica Structures: Am. Mineralogist, vol. 33, pp. 751 ff., 
1948. 

^ Barshad, J., Vermiculite and Its Relations to Biotite as Revealed by Base Exchange Reaction, 
X-ray Analysis, Differential Thermal Curves, and Water Content: Am. Mineralogist, vol 33, pp. 
655 ff., 1948. 

'* Buerger, M. J., op. cit. 



Subsurface Geologic Methods 




Plate 9. Typical shale patterns. (1) Puente shale formation, Cohon, California; 
(2) Salinas shale formation, Santa Barbara County, California; (3) Monterey 
shale formation, Santa Cruz, California; (4) Mowry shale formation, Casper, 
Wyoming; (5) Black Diamond shale, Metalene Falls, Washington; (6) kerogen 
(oil shale). Green River formation. Rifle, Colorado; (7) shale from Benton 
formation, Golden, Colorado; (8) illite-glauconite shale, Granby, Colorado. 



Subsurface Laboratory Methods 239 

materials must be expressed as a range rather than as a definite value. 
Such variations in chemical composition with resulting changes in lattice 
dimensions cause differences in line intensities, shifting of lines, appear- 
ance or disappearance of lines, and other changes in X-ray-diffraction 
patterns. Consequently, it is not possible to establish a standard pattern 
for some minerals, as has been attempted in the card-index system. For 
proper identification of compounds with variable chemical composition, 
isomorphism and phase relationships must always be considered. For 
such compounds, complete knowledge of the identity and structure will 
be obtained only from simultaneous consideration of chemical compo- 
sition, crystallography, physical and physical-chemical properties, and 
X-ray-diffraction data. 

Occasionally, evidence of atomic replacement within crystals is hardly 
detectable in the X-ray-diffraction patterns, ^specially where the unit-cell 
parameter is dependent on a certain kind or kinds of atoms or ions which 
form a rigid geometric-space packing, with the other atoms or ions fitted 
loosely into the holes of the structure. Substitutions of the latter type of 
atoms or ions may cause little if any change in the lattice parameters of 
the crystal. 

Applications 

The X-ray-diffraction methods of analysis of geologic materials can 
be used in subsurface investigations to supply the geologist with infor- 
mation not otherwise obtainable, to furnish the petroleum engineer with 
precise knowledge of the composition and certain properties of reservoir 
rocks, and to trace mineralogic and structural changes of importance in 
problems of sedimentation and sedimentary petrology. 

The precise identification of mineralogic composition made possible 
by the X-ray-diffraction method will permit the correlation of formations 
where other data are lacking, or may prevent erroneous correlation based 
on unreliable information. Identification of the kind and amount of minor 
constituents in apparently homogeneous, thick formations may subdivide 
the sequence in such a manner as to demonstrate the stratigraphic rela- 
tionship to similar formations occurring elsewhere. 

The analysis of reservoir rocks by X-ray-diffraction may reveal de- 
tails of composition otherwise overlooked. In particular, the kind and 
amount of interstitial clay may critically control effective porosity and 
permeability of formations by changes in hydration and degree of floccu- 
lation, as a consequence of change in the solutions saturating the rock. 
Flocculation or deflocculation and hydration or dehydration of clays are 
controlled by their mineralogy as well as by their environmental changes. 
Hence, the susceptibility of clays to change during the water-flooding or 
other secondary-recovery programs can be detected by X-ray-diffraction 
analysis of reservoir rocks. 

The geologist and engineer will find that X-ray-diffraction methods 



240 Subsurface Geologic Methods 

increase the reliability of geologic logging. The method supplements 
petrographic techniques of logging drill core, making possible quick and 
precise identification of even exceedingly fine-grained types and complex 
mixtures. In addition, the method can supply basic data on petrography 
and mineralogy necessary to interpret completely the electric and gamma 
radiation logs of drill holes. Both engineers and geologists are finding 
that the X-ray method of analysis is a powerful tool in the identification 
of potentially unsound materials in foundation strata or construction 
materials proposed for use in dams, powerhouses, buildings, and other 
large engineering works. 

Finally, X-ray-diffraction analysis, both of geologic materials col- 
lected in the field from outcrops and cores and of synthetic materials in 
the laboratory, will yield detailed knowledge of processes involved in 
deposition, consolidation, and induration of sediments. The methods of 
X-ray-diffraction analysis are unsurpassed in effectiveness and efficiency 
in the tracing of progressive changes in mineralogy and structure of 
materials. Application of these methods will demonstrate the process 
of recrystallization during consolidation and induration, such as may 
occur in unstable minerals like clays, and the formation of new minerals, 
such as feldspar, mica, and zeolites. Only when these and related pro- 
cesses are understood will the conditions of petroleum formation, migra- 
tion, accumulation, and production be understood fully. 

The versatility and adaptability of X-ray-diffraction methods have 
justified recognition by the petroleum geologist, engineer, and chemist. 
For one problem the methods may afford merely a valued supplement 
to other techniques; for another problem the methods may be indispens- 
able to a successful solution. Consequently, the supervisor of subsurface in- 
vestigations should be cognizant of the potentialities of the X-ray-diffrac- 
tion methods so that they will be used when and as required by the 
nature of the problems to be solved. 

MULTIPLE-DIFFERENTIAL THERMAL ANALYSIS 
PAUL F. KERR and J. L. KULP 

Differential thermal analysis provides a useful technique for the study 
of specific minerals or mineral groups with distinctive heating curves. 
The method is suitable for both qualitative and semiquantitative studies 
of the clay minerals, the hydrous oxides of iron, aluminum and manganese, 
the carbonates, the zeolites, and a goodly number of other minerals. In 
general, the method applies to substances that yield characteristic peaks 
in the differential thermal curves. 

In this technique a dual-terminal thermocouple is employed. One 
terminal is inserted in an inert material which does not undergo exo- 
thermic or endothermic reaction through the temperature interval to be 
studied. The other is placed in the mineral or mixtures of minerals under 



Subsurface Laboratory Methods 241 

test. With a constant heating rate a thermal reaction in the sample will be 
recorded as a deviation from the straight-line plot of temperature differ- 
ence against temperature. This deviation is dependent upon the nature 
of the heat change for its direction and amplitude. Peaks may be due to 
loss of either absorbed or lattice water, decomposition, or changes in 
crystal structure. They are characteristic for most thermally active min- 
erals. Mixtures show a composite curve of the effects of the individual 
components in their proper proportion. 

Although the original work on thermal analysis was done by Le 
Chatelier in 1887, it was not until the late 1930's that the method began 
to be used for semiquantitative study of clay minerals. In recent years 
studies have been made at the National Bureau of Standards,^'^ the Massa- 
chusetts Institute of Technology,^^ the United States Geological Survey,^^ 
the Illinois Geological Survey,'^^ the Bureau of Plant Industry,*'^ and 
various United States Bureau of Mines research laboratories.^^ 

Publications resulting from these studies emphasize the value of dif- 
ferential thermal analysis as a supplementary method coordinated with 
chemical, optical, and X-ray methods in studying clay minerals. X-ray 
data may have certain advantages in indicating a general clay-mineral 
group. Thermal-analysis curves, on the other hand, may contribute quan- 
titative data on mixtures not readily available from X-ray-dififraction 
studies. Also, substitution in the clay-mineral lattice is frequently more 
apparent in the peak shifts of thermal curves than in X-ray patterns that 
frequently lack suitable definition. In combination, the two methods offer 
a solution to many complex problems in the study of clays. 

The authors wish to acknowledge the helpful criticisms of the manu- 
script received from R. E. Grim, the Illinois Geological Survey; Ben B. 
Cox and Duncan McConnell, the Gulf Research and Development Labora- 
tories; M. L. Fuller, T. L. Hurst, L. D. Fetterolf, and D. G. Brubaker, the 
New Jersey Zinc Company; Robert Rowan and R. H. Sherman, the Creole 
Petroleum Corporation; and Parke A. Dickey, the Carter Oil Company. 

The Apparatus 

The use of thermal analysis in the study of argillic alteration of a 
mineralized area or a stratigraphic-correlation problem requires the test- 
ing of hundreds of samples. This has involved a tedious laboratory pro- 
cedure in the forms of apparatus described in the literature,^^ ^^ where a 

^' Insley, H., and EweO, R. H., Thermal Behavior of Kaolin Minerals: Nat. Bur. Standards Jour. 
Research, vol. 14, pp. 615-627, 193S. 

^^ Norton, F. H., Critical Study of the Differential Thermal Method for the Identification of the Clay 
Minerals: Am. Ceram. Soc. Jour., vol. ?2, pp. 54-63, 1939. 

^^ Alexander, L. T., et al.. Relationship of the Clay Minerals Halloysite and Endellite: Am. Min- 
eralogist, vol. 28, pp. 1-18. 1943. 

*" Grim, R. E., and Rowland, R. A., Differential Thermal Analysis of Clay Minerals and Other 
Hydrous Materials: Am. Mineralogist, vol. 27, pp. 746-761; 801-818, 1942. 

'^ Hendricks, S. B., Goldrich, S. S., and Nelson, R. A., On a Portable Differential Thermal Outfit: 
Econ. Geology, vol. 41, p. 41, 1946. , 

*- Speil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, Differential Thermal Analysis, 
Its Application to Clays a-'d other Aluminus Minerals: U. S. Bur. Mines Tech. Paper 664, 81 pp., 1945. 

°^ Speil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 

'■^ Norton, F. H., op. cit. 



242 



Subsurface Geologic Methods 



single sample is run at a time. Since each run requires several hours in- 
cluding cooling time, a maximum of about three samples a day may be 
analyzed. To overcome this difi&culty, as well as to provide a simultaneous 




Figure 100. Complete multiple-differential thermal-analysis unit. 



comparative record, a multiple-thermal-analysis unit was designed.^^ The 
various parts of the equipment were assembled late in November 1946 

°* Kulp, J. L., and Kerr, P. F., Multiple Thermal Analyses: Science, vol. 105, no. 2729, p. 413, 1947. 



Subsurface Laboratory Methods 243 

and were placed in operation about January 1, 1947, and approximately 
1,500 samples had been run by August 1, 1947. 

Figure 100 shows the apparatus as set up the mineralogical labora- 
tory at Columbia University. For purposes of description, the apparatus 
may be conveniently divided into four parts: the furnace, the sample 
holder, the program controller, and the multirecorder. 

The furnace is a Hoskins 305 electrical-resistance furnace into which 
an alundum tube (If inches inside diameter by 12 inches with a three- 
sixteenths-inch wall) is inserted to diffuse the heat and to insulate the 
metal specimen holder from the heater coils. The furnace is mounted 
vertically on a track and can be raised or lowered over the specimen holder 
by means of counterweights attached to two cables over pulleys. 

The specimen holder (fig. 101) is drilled from a cylindrical block of 

PLAN VIEW 




Figure 101. Nickel specimen holder. 

chrome-nickel steel If inches outside diameter and one inch in height. 
Both pure nickel and chrome-nickel steel have been used, but the latter has 
similar heat conductivity and is less subject to scaling. The six samples 
to be tested are loaded in the outer holes numbered 1 to 6, while the 
inner holes 1', 2', 3' are used for inert material, which is ordinarily puri- 
fied alundum manufactured by the Norton Company. The dashed lines 
indicate the connections between the two terminals of the chrome-alumel 
differential thermocouples. Thus one hole containing alundum is suflB- 
cient for the inert side of two differential couples. Chrome-alumel couples, 
BXS22, were used for maximum electromotive-force generation and were 
found to be substantial. The dots a indicate the position of the tempera- 
ture-recording thermocouples. The terminals of these couples are adjusted 
to the same height as the differential couples in the samples. The sample 
and alundum holes are one-fourth inch in diameter and three-eighths inch 
deep. 

The chrome-nickel-steel block is supported by an alundum tube (1| 



244 



Subsurface Geologic Methods 



inches inside diameter by 6^ inches with a one-fourth-inch wall) and sup- 
ports a cylindrical cover of solid nickel half an inch thick, placed on the 
block to shield the samples from direct radiation. Two complete units of 
sample holder and thermocouples were prepared. Thus, if a break occurs 
in one thermocouple, the entire unit may be recovered without delay and 
a replacement connected. The next run may thus be carried out without 
loss of time for repairs. 

The program controller is a special Leeds and Northrup "Micro- 
max," which is connected to one of the four possible temperature-record- 
ing thermocouples by way of a rotary selector switch. This unit is rated 
to raise or lower the temperature of the sample at any desired rate from 
0° to 50° C. a minute. It will also automatically hold the samples at 



1.5 V 



20ca 



-wm- 



o.4n 

-ww- 



J. *00Q 

-vvvvv-r^wv^T"Vvvy^T-vvw-r-vww-r^w^'-^-^^^ 



b 



•</6 MV* M/S MV« M/B MV« •t/6 MV« 



ifl 



. 10 Record 

.to Thsrmocoupit 



Figure 102. Potentiometer circuit for spreading records. The 
unit is placed in series with one head of each thermo- 
couple. The desired position for each couple is achieved 
by connecting across appropriate terminals from a to g. In 
the diagram, connection on a and b would add a con- 
stant 1/6 mv. to the base line of the differential thermal 



any desired temperature when that temperature is reached. The pen record 
indicates the temperature of the sample. The controller, when properly 
adjusted, gives a linear heating curve. 

The recorder for the differential thermocouples is a Leeds and North- 
rup "Speedomax," a six-point, high-speed, high-sensitivity electronic re- 
corder with a maximum range of three millivolts. The chart of this re- 
corder is synchronized with the chart containing the temperature record 
on the program controller. This recorder is sensitive to 0.1° C. differ- 
ential temperature, which, with the present specimen holder, gives a peak 
one centimeter in amplitude for the alpha-beta quartz change. Experi- 
mentation on increasing sensitivity with accessory devices is in progress. 
However, it should be pointed out that, beyond a certain limit of sensi- 
tivity, thermal gradients, geometry, thermocouple defects, and other un- 
known factors cause prohibitive irregularities in the base line. The pres- 



Subsurface Laboratory Methods 245 

ent equipment yields curves that are reproducible to a degree or so in 
peak temperature and to five percent of the peak amplitude under nor- 
mal conditions. 

Since all the differential thermocouples print at zero millivolts where 
there is no reaction taking place, it is desirable to spread the six records. 
This is done by a simple potentiometer circuit (fig. 102), which places the 
base line of each record about one-sixth of a millivolt from its nearest 
neighbors. The exact separation desired is achieved by adjusting a 200- 
ohm resistance in the battery circuit. It is also desirable to have certain 
sensitivity scales available, since some of the reaction minerals such as 
alunite, jarosite, kaolinite, and carbonates may extend beyond the chart 
on high sensitivity. Because this type of recorder measures the electromo- 
tive force of the thermocouple, a simple voltage divided with proportionate 
resistances is efficient for obtaining one-half, one-third, or any other pre- 
determined fraction of the generated electromotive force. 

Finally, there are two solenoid pens in series, one attached to the 
edge of each recorder. By means of a button switch, the solenoids are 
simultaneously activated, thus marking both records at the same time. 
Since the temperature at that instant can be read from the program- 
controller record, the temperature of the six records is also known and 
can be written on the multirecord chart at the completion of the run. 

The advantages of this equipment are worthy of note. One of the 
greatest is the multiple-record feature, by means of which with three 
runs eighteen samples may be tested conveniently in an eight-hour day. 
Also significant is the reduction in the number of potential variables in 
using six samples under the same heating conditions. This is important 
when runs of quantitative mixtures are compared. The unit is compact, 
it does not require a darkened room for operation as in the photographic 
recording methods, and the results are immediately observable. The chief 
disadvantage lies in the necessity for applying minor corrections to each 
curve. 

Procedure 

The samples to be tested by the differential-thermal-analysis appa- 
ratus are passed through a 50-mesh screen and packed to finger tightness 
around the differential thermocouple. No pretreatment is given for an 
ordinary run. It has been found by experimentation, as reported by 
others, that any attempt to attain equilibrium with a specified humidity 
merely alters the initial absorbed-water peaks (100°-200° C), the ampli- 
tudes of which are usually not used for quantitative analysis. Ordinarily 
weighing has been found to be unnecessary, and reproducible curves may 
be obtained for the same pure substance with finger-tight packing with a 
close-fitting metal plunger to a constant level. In special cases attention 
must be given to the problems of particle size, weight, and humidity. 

After the samples are loaded, the cover is placed on the specimen 



246 



Subsurface Geologic Methods 



holder, and the two charts are synchronized, the furnace is started. The 
heating rate has been standardized at 12° a minute, as this gives sensi- 
tive control, produces adequately sharp peaks, and is close to the heat- 
ing rate used by a number of other workers in this field. The record 
is made from 100° to 1,050° C. At the beginning and end of the run 
the button switch activating the solenoid pens is pushed, thus fixing the 
temperature on the multiple-differential thermocouple record. When 
1,050° C. is reached, the furnace is raised from the specimen holder, and 







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* 






°N 


y^c 










\ 




o 


5 






> 


r 


i 












K 


«: 










• 


V 










< 


^ 








b 




Q 








' 1 


•s 



) 200 SOO 400 500 600 700 

Dtgrttt Ctntigroda 
Figure 103. Theoretical thermal curves. 



the samples are removed by compressed air while they are still hot. This 
procedure prevents caking, which occurs in certain specimens upon 
cooling. 

The temperature thermocouples are calibrated and recalibrated occa- 
sionally with the alpha-beta quartz change. It has been found after trying 
many thermocouples that the quartz-inversion peak occurs on a differential 
curve within 5° of 579° C. (This is higher than the equilibrium value.) 



Subsurface Laboratory Methods 247 

The reproducibility has also been observed with the standard Georgia 
kaolinite endothermic and exothermic peaks. Since this is consistent with 
data in the literature and since the change in peak temperature of pure 
hydrous minerals may easily vary 5° C, more precise calibration has 
been considered unnecessary. Different sample blocks, thermocouples, and 
furnace windings produce no change in peak temperature greater than 
5° C. 

Although the thermocouples are made as similar as possible and ad- 
justed to approximately the same heights in the sample holes, the sensi- 
tivity varies slightly. Therefore, after replacement of one specimen holder 
and the corresponding thermocouples by another, the first run is usually 
made with standard Georgia kaolinite in all sample holes. This indicates 
the relative sensitivity of the various thermocouples. It has been found 
that these relative sensitivities remain essentially constant for the life of 
the thermocouples unless the height of the thermocouple is changed as a 
result of rough handling. 

All curves included in this description are based on the same sensi- 
tivity for direct comparison. It has been found convenient to plot the 
differential-thermal curve so that an exothermic peak is upward, while an 
endothermic reaction is represented by a deviation downward from the 
base-line curve. 

Theory 

The theory of differential thermal analysis has been presented by 
Speil.^^ The following account, modified and corrected,^^ is included to 
aid in introducing the present studies. 

Figure 103 compares two methods of dehydrating a clay mineral. The 
static method produces the equilibrium-dehydration curve, while the dy- 
namic method gives the differential-thermal curve. In the first instance, 
the sample is held at each successively higher temperature until it has 
reached equilibrium. In the second, the sample is heated at a constant rate, 
thus extending the dehydration over a longer temperature range. Since 
the thermal curve is a differential function, it depends only on those ef- 
fects that do not occur simultaneously and equally in the specimen and 
the inert material. Hence, there are only two thermal effects to consider, 
the differential flow of heat from the block to or from the thermocouple in 
the center of the sample and the heat of the thermal reaction. The differ- 
ential-thermal curve of figure 103 represents an endothermic reaction. Be- 
low temperature a the heat inflow to both thermocouples, sample and 
inert material, is the same, and no difference in temperature is recorded. 
At a the reaction starts absorbing heat from its surroundings, making the 
sample couple cooler than the alundum couple. This effect increases until 
at b the rate of heat absorption by the chemical reaction equals the rate 

^' Speil, Sidney, Berkelkamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 

*' Error in Spiel's derivation pointed out by Dr. D. G. Brubaker, N. J. Zinc Co., Palmerton, P». 



248 Subsurface Geologic Methods 

of differential-heat conductivity into the clay specimen. The rate of heat 
absorption then continues to decrease more rapidly than the inflow of heat 
from the block. At this point d between h and c, the reaction ceases. How- 
ever, since this point cannot be established exactly, a and c are usually 
chosen as limits. 

Under static conditions the heat effect would cause a rise in temper- 
ature AT^s of a specimen given by: 

where M= the mass of the reactive mineral, 
H = xhe specific heat reaction, 
Mo — the total mass of the specimen, and 
C = the mean specific heat of the specimen. 
However, the heat flow from the nickel block towards the centers of the 
two sample cavities must be taken into account. 

For any point between a and c, the simplified equation describing the 
changes in heat content of the thermally active constituent is: 

Mj^^Jf+gA;J {T-T)dt=MoC{T-Ta) (2) 

(A) (B) (C) 

for the inert sample: 

8-Ti' r{T,-r)di=Mo'C{r-T,n 

J a 

(B)' (C)' 



(3) 



where t = time. 

Mo = the total mass of the test specimen, 
Mo = the total mass of the alundum, 

C'= the mean specific heat of the test specimen, 

C'= the mean specific heat of the alundum, 

k = the conductivity of the specimen, 

k'= the conductivity of the alundum, 

g = the geometric-shape constant. 
To = the temperature of the nickel block, 
Ta = the temperature at the center of the sample at time 

T = a, 
Ta — the temperature at the center of the alundum at time. 
T = a, 

T = the temperature at the center of the sample, 

T'= the temperature at the center of the alundum. 



Subsurface Laboratory Methods 249 

Factor A defines the quantity of heat added to or subtracted from 

dH , 

the test specimen owing to reaction. In an exothermic reaction is posi- 

dt 

tive. Factor B ^^ defines the quantity of heat absorbed by the specimen ^^ 
A + B =C, because at any point x along the differential-thermal curve, 
the amount of heat used in raising the temperature of the specimen must 
equal the amount brought in by flow from the metal block plus the 
amount added or subtracted by the reaction. 

In the event sample factor a does not exist, the heat which flows in 
B' must equal the heat used in raising the temperature of the specimen C. 
Let C=C + AC 

and k'=k+ Ak 

Also in the experimental procedure Mo=Mo' within the error of 
measurement. Subtracting (3) from (2) and rearranging gives: 

M r^dt+gk Civ-Ddt-gAK r{To-r)dt 

J adt J a J a 

=Mo\C[{T-Ta)-{r-Ta')]-AC[r-Ta']\ ^^^ 

=Mo\D[{T-r)-Ta-Ta')]-AC[T'-Ta']\ 

As T'— r=r=temperature indicated by the differential thermocouple, the 
equation can be considerably simplified by assuming that the term con- 
taining {To—T'), C, and K are small in comparison with other terms. By 
using a and c as integration limits : 

M r^dt+g . k (\Tdt=MoC[ {T-Te') -{Ta-Tr!) 1 (5) 

J adt J a) 

but to a close approximation ^^ 

and 

J<^dH 
~dt=MAH, 

the total heat of reaction 

,o,ES^ = r\Tdt. (6) 

The last expression is proportional to the area enclosed by a straight 
line from a to c and the curve ahc, if the deviation from the base line 
is a linear function of the differential temperature. It is proportional, 
therefore, to the percentage of reacting material in a given weight of 

°^ The temperature gradient in the chrome-nickel steel can be neglected as the thermal conductivity 
of the metal is so much greater than that of the refractory sample to be tested. 

°' (Ta — Ta) and (Tc- -Tc) will equal zero for specimen holders in which the test and alundnm 
holes are symmetrically spaced relative to the heat source. In the present concentric type of spacing 
\Ta — Ta) = (Tc — Tc) within the error of measurement. 



250 Subsurface Geologic Methods 

suits obtainable. It is believed that they agree substantially with those 
sample. This forms the basis for the quantitative use of differential ther- 
mal analysis. The linear relationship holds reasonably well. More exact 
determinations of comparatively simple systems can be made by running 
known mixtures and preparing a calibration curve of area versus per- 
centage of each component. 

The derivation above neglects the differential terms and the tempera- 
ture gradient in the sample. It shows that the area under the curve is a 
measure of the total heat effect. The area is also considered independent 
of the specific heat. This factor, however, actually does affect the shape 
of the peak and may change the area slightly. For many purposes the 
approximate relationships are sufl&cient. 

Qualitative Applications 

The various clay minerals yield suJG&ciently different peaks to make 
the differential-thermal-analysis method particularly useful. When a speci- 
men is relatively pure, preliminary identification by thermal curves is 
frequently comparatively simple. In addition, two-component mixtures 
are often resolved and at times even three-component mixtures. If, how- 
ever, mixtures become too complex, only one or possibly two of the major 
components may be identified. 

The multiple-thermal-analysis apparatus makes possible a rapid, 
widespread survey of the groups of minerals that can be identified by this 
procedure. The thermal curves given here are representative of the re- 
of the other workers on record. Illustrative curves are shown for the 
kaolinite and montmorillonite groups along with the hydrous oxides of 
aluminum and iron and some sulphates and carbonates. 

The kaolin minerals (figs. 104, 105, and 106) are characterized by a 
large endothermic peak ranging from 550 to 700° C, owing to the 
decomposition of the kaolinite lattice into amorphous silica and alumina 
and a sharp exothermic peak of 980° C. caused by the recrystallization 
of amorphous alumina to gamma alumina. 

Thermal curves of dickite from several localities are shown in figure 
104. A number of the samples illustrated have been studied in connection 
with other investigations to such an extent that they may be considered 
representative of this clay mineral. The sample from Red Mountain, 
Colorado, was ground, and curves were run to compare 100-200-mesh, 
200-300-mesh, and smaller than 300-mesh material. It is interesting to 
note that an ordinary specimen from St. Peter's dome yields a curve 
similar to Red Mountain dickite ground to minus-300 mesh. It is evident 
that particle size is a factor to be considered. 

As the degree of orderliness in the superposition of the kaolin layers 
decreases from dickite through kaolinite to halloysite, the endothermic 
peak shifts downward in temperature, indicating less stability of the lat- 
tice. The disorder reaches a sufficient extent in the case of halloysite to 



Subsurface Laboratory Methods 



251 



D I C KIT E 



iqo 



Degrees Centigrade 
300 §50 700^ 



r-¥ 



Freiburfl.Germony | 1 1 T 

(Nocrltt) 



Son Juonito, 

Mexico (A) I I I T 



Son Juonito, 



Mexico (B) 



Schuylkill Co, 
Pennsy I vonio 



Red Mounlain, 
Colorado 

(100-200 ll«lll) 



Red Mounloin, 
Colorado 

1200-300 Mtik) 



Red Mounloin, 
Colorodo 

k< 300 Hxk) 



Si. Peter't Dome, | 1- 

Colorodo 



100 



_J — I I I L_ 

300 500 



I I 




700 900 

Dtgrees Centigrode 



.1.1 



1100 



Figure 104. Thermal curves of dickite from several localities. Curves 5, 6, and 7 
indicate effect of grinding on dickite from Red Mountain, Colorado. 



252 Subsurface Geologic Methods 

permit absorption of some water between the lattice layers. This accounts : 
for the minor endothermic peak at about 150° C. This peak is greatly en- , 
larged in the case of endellite, the more hydrous form of halloysite. En- 
dellite yields water to form halloysite under 100° C. Allophane has been 
considered in the kaolin group ^*^ presumably because of the sharp 980° C. 
exothermic peak corresponding to the formation of the gamma alumina 
and a rough agreement of chemical analyses. The kaolin group is best 
delimited by the unique lattice type, which is observed in nacrite, dickite, 
kaolinite, and halloysite. 

Figure 105 contains a series of kaolinite curves. The variations in 
the shape of the endothermic peaks are probably due to differences in clay- 
mineral particle-size distribution. The samples with a narrow range of 
particle size appear to give the sharpest peaks. Since the total heat evolved 
is dependent only on the concentration of reactive molecules present 
around the thermocouple, the area under the curve should be roughly 
constant. It is evident from the set of curves that the differences in the 
shapes of the curves are not great, and, hence, the amplitudes are essen- 
tially the same. The specimens from Dry Branch, Georgia; Cornwall, 
England; Newman Pit, California; Franklin, North Carolina; and Santa 
Rita, New Mexico, give characteristic kaolinite X-ray patterns. The 
Georgia material was used as a standard for comparison. 

The samples were prepared by passing the kaolinite through a 50- 
mesh screen. One specimen (not shown) of Georgia kaolin fines ob- 
tained by gravity separation exhibits a kaolinite curve with an endother- 
mic peak depressed slightly and lowered in temperature about 10°. The 
980° C. exothermic peak of this material also shows a slight shift to lower 
temperatures. This probably is because of the finer-particle-size mate- 
rial, which is in a less stable state with a correspondingly lower tempera- 
ture of recrystallization, 

A private communication from R. E. Grim indicates that some kao- 
linite samples give curves with upward swings in the thermal record be- 
tween the large endothermic reaction and the final exothermic reaction. 
These same kaolinite samples also give a slight endothermic dip just be- 
fore the final exothermic peak. Of the few kaolinite curves shown in figure 
105, only the Marysville, Utah, and Northwest, New Mexico, samples in- 
dicate the slight endothermic peak mentioned above. 

Figure 106 contains typical thermal curves of halloysite, endellite, 
and allophane. The first four halloysite samples are believed to be of 
high purity. X-ray-dij6Fraction photographs have been obtained from 
these four specimens and display the lines of halloysite. The thermal 
curves are similar to those of kaolinite with two significant differences, 
the small endothermic peak at 150° C. due to adsorbed water, and the shift 
in the main endothermic peak to about 570° C. Grim "^^ claims that halloy- 
site does not have a lower temperature for the main endothermic peak than 



'" Speil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 

"Grim, R. E., Modern Concepts of Clay Minerals: Jour. Geology, vol. 50, pp. 225-275, 1942. 



Subsurface Laboratory Methods 



253 



Kaoli n ite 



LOCALITY 



Degrees Centigrode 



100 



P ' I 'T ' I 'T ' I ' T 



UKroine, 
US S.R. 



CornMOll, , 

Englond ' 



Ff onkli n. 
No. Corolino 



Santo Rita, 
Ntw Mtxico 



Mor y s vole 
Uloh 



I ^p^ I I "< ?° 



Dry Brooch, 

Georgio ' '' 



H 1 ^ 



i 1- 



Northwest, 

New (Mexico I 1 1 r- 



Newmon Pit, , , 

Collfornio I 1 1 r- 




H 1 f 



100 



I ' I ■ I ■ I 




^u 



i h 




-^ h 




J_J 1 ' ' ' ' I L 



300 500 700 900 

Degrees Centigrode 



ilOO 



Figure 105. Thermal curves of eight specimens of kaolinite from different localities, 

all similar in character. 



254 Subsurface Geologic Methods 

kaolinite. Other workers '^- ^^ show evidence of the lower temperature of 
the halloysite peak. In this laboratory eight typical halloysite specimens 
yield endothermic peaks at 575° ±10° C. On the other hand, the kao- 
linite samples examined show endothermic peaks at 600° C. or above. 
This difference is well above the limits of experimental error. One pos- 
sible complication is noted in the case of very fine kaolinite. Here the 
605° C. peak is shifted downward toward the halloysite peak. However, 
the fine kaolinite does not have so low an endothermic peak as does 
halloysite, the amplitude and the shape of the 600° C. peak is altered, 
and the 980° C. exothermic peak is shifted down the temperature scale. 

These data are not sufficiently conclusive to establish the range of 
the endothermic peak of kaolinite. The structure variations in halloysite 
and kaolinite and the correlation with the thermal phenomena require 
further investigation. 

The halloysite from Bedford, Indiana, contains a small amount of 
gibbsite. The endellite from Bedford is typical, showing the halloysite 
curve with a greatly increased low-temperature endothermic peak. The 
last two curves in figure 106 are typical of allophane. 

Figure 107 contains the thermal curves of certain three-layer lattice 
minerals. Montmorillonite furnishes a broad classification for a certain 
crystal structure, but with wide substitution possibilities in the lattice. 
An excellent paper by Ross and Hendricks '^^ has contributed to the clari- 
fication of this group. The thermal curve of the Polkville, Mississippi, 
material exhibits the previously recognized low-temperature doublet, the 
two high-temperature endothermic peaks, and the final high-temperature 
exothermic peak. The amplitude of the doublet is dependent to a large 
extent on the humidity conditions before thermal analysis. Hendricks and 
others "^^ pointed out that the shape of these peaks was due to the quantity 
of adsorbed water and the type of adsorbed cation between the three-layer 
units. The high-temperature endothermic peaks occur variably between 
the limits of 550° C. and 1,000° C. This is probably to be attributed 
to substitution within the layer itself. The temperature of the peaks has 
not as yet been correlated with chemical analysis. This is now being 
investigated. The high-temperature exothermic peak is dependent in part 
on the substitution of iron for aluminum within the layer. The substitu- 
tions in the montmorillonite lattice are perhaps more apparent from the 
shifts in the thermal-curve peaks than the shifts in the lines of the diffuse 
X-ray-diffraction patterns. 

The curves of specimens from Ventura, California; Wisconsin; 
Texas; and Rideout, Utah, are typical of montmorillonite. The "meta- 

'^ Spiel, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 

" Norton, F. H., Analysis of High-Alumina Clays by the Thermal Method: Am. Ceram. Soc. Jour., 
vol. 23, pp. 281-282, 1940. 

'* Ross, C. S., and Hendricks, S. B., Minerals of the Montmorillonite Group: U. S. Geol. Survey 
Prof. Paper 205-B, 1945. 

'^ Hendricks, St B., Nelson, R. A., and Alexander, L. T., Hydration Mechanism of the Clay Mineral 
Montmorillonite Saturated with Various Cations: Am. Chem. Soc. Jour., vol. 62, pp. 1457-1464, 1940. 



Subsurface Laboratory Methods 



255 



Halloysite. Endellite, Allophane 



Degrees Centigrode 
'pp. , .30°. . . 5 00 700 900 nOO 

r I ' I ' I ' I ' I ' I ' I ' I ' I ' 1 




Ui«g«, Btlgium |\/f 



Tinfie.Utoh \^^ 1 1— -J ^ \^ 1 I I 



Gol«na, Konsos [■v./|' 



Outens, 
Kentucky ^"xA ' 



Bfdford, Indiono 



Bedford, Indiono I 



Missouri k I 



Styrio I ( 




1 — r 



-I \r- 



J L 



MINERAL, 
I HoUoysite 

I Holloyslte 
I Holloysite 



J- 1 '\\ I Holloysite 



x 



Holloy site 

ond Gibbsite 



Endellite 



I I Alloptione 



I I I 1^1 Aiiophone 



I .■ I ■ .I,' I I I I I . I . I . I ■ I . I 
30 500 500 ^ht 560 — lie 

Degrees Centigrade 



100 



100 



Figure 106. Thermal curves of halloysite mixed with some gibbsite, endellite, and 

allophane. 



256 Subsurface Geologic Methods 

bentonite" from Highbridge, Kentucky, contains potash, but the thermal 
curve indicates a material more like montmorillonite than hydromica or 
illite, in which the first high-temperature endothermic peak occurs at 600° 
C. or lower. The specimen from Candelaria, Nevada, shows a thermal 
curve more characteristic of hydromica, while that from Transylvania 
appears to be a mixture of montmorillonite and hydromica (illite) . 

The last three curves of this set are from specimens labelled "sapon- 
ite," the high-magnesium montmorillonite clay. This mineral shows a 
distinct double peak in the neighborhood of 800 to 850° C. All of these 
specimens give saponite X-ray patterns. 

Figure 108 shows thermal curves of gibbsite, diaspore, brucite, and 
goethite. All of these specimens were checked by X-ray diffraction. The 
curves for gibbsite agree with those in the literature, which show the main 
endothermic peak to occur from 330 to 350° C. Although Speil's 
sample "^^ does not show the lower-temperature minor endothermic peak, 
the others do. This may be assumed as due to the high purity of Speil's 
sample. Pack and Davies '^^ ascribe the initial minor endothermic peak to 
cliachite. The specimen from P050S de Caldos appears to contain a small 
amount of kaolinite. 

The diaspore labelled "white bauxite" from China is apparently un- 
usually uniform in grain size. It powders readily on crushing the sample, 
making grinding unnecessary. Conversely, the coarsely crystalline diaspore 
from Chester, Massachusetts, requires considerable grinding. The result- 
ing material evidently has a large grain-size distribution, as indicated by 
the shape of the curve. Apparently both of these specimens are of high 
purity. 

Typical well-crystallized brucite specimens from Texas, Pennsylvania, 
and Gabbs, Nevada, give thermal curves which correspond. Similarly 
curves from specimens of goethite from the Lake Superior copper district 
and Roxbury, Connecticut, agree with each other. 

Curves for alunite and jarosite are shown in figure 109. These miner- 
als display prominent peaks that are distinctive and can be detected in the 
presence of foreign materials. The specimens from Bulledehah and Bar- 
ranca probably contain inert impurities such as sericite that have depressed 
the peaks. Those from Santa Rita, Hyagoken, Los Lamentos, and Tintic 
have been checked by means of X rays. 

Figure 110 shows some preliminary carbonate curves. These are con- 
sistent with themselves and indicate the possible use of thermal analysis 
for quantitative studies of carbonate rocks. Siderite ^^ and rhodochrosite 
yield exothermic oxidation "domes" owing to the reaction with the oxygen 
of the air of the lower-valence oxide produced in the carbonate decom- 



'* Spoil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 
" Speil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, idem. 

" Kerr, P. F., and Kulp, J. L., Difierential Thermal Analysis of Siderite: Am. Mineralogist, vol. 32, 
p. 678, 1947. 



Subsurface Laboratory Methods 



257 



Certain 3-layer Lattice Minerals 



0«gr«t» Ctntlgrod* 



Conddtrio, Nivodo 



Trontylvonie 



N««dl«t. Colif. 



Montrtol, Ou*b«e 



Gioigoa, Scotlond 




Hydromieo ? 
(IlliU) 



Monrmorillonit*- 
i and Hydromico? 
(Illit*) 




1 ■ I Soponilt 



I I I J SoponlH 



I Soponil* 



■ I ■ I L 



100 300 



■Jill Lt 

500 700 



Ocor*** Canllgrod* 



I ■ Xa I ■ I 



900 TTOO 



Figure 107. Several thermal curves of montmorillonite, saponite, and hydromica 
(metabentonite and illite) are illustrated. 



258 



Subsurface Geologic Methods 



Certain Hydrous Oxides 

Degrees Centigrade 



Pocos de Coldot, 
Bronl 



Richmond , Mo ss. 



"White Bouiile" 



Chester, IMoss I 1 h 



Teios, Po I 1 1 ^ 



Gobbs.Nevodo i ^ 1- 



Lohe Superior I I 



Roi bury , Cortn 




I Goethite 



Degrees Centigrode 



Figure 108. Curves of gibbsite, diaspore, brucite, and goethite shown for comparison. 



Subsurface Laboratory Methods 



259 



Alunite and Jarosite 



LOCALITY 



Sonto Rito. 
N«« Mfiico 



100 

r- 



Bulltdthoh, 
N S Wol«» 



Sonto Morlo Mine, 
Jtlordtno, Moiico 



Lot Lomtntet, 
Chlhuohuo, 
Mtiico 



Tinfic, 
Uloh 



Degrees Centigrade 

500 700 900 



1 — ' — r 



Hyogoken, ^^ 

</opon r 1 r h 



Tolfo. 
Itoly 



Borroneo. . | i_ 

JorotO, Spoilt 



1100 




Vi 



I.I.I 



100 



300 



_L_I . ' ■ I ' I . I ^ I ■ I 

500 700 900 lie 

Degrees Centigrode 



Figure 109. Thermal curves for alunite and jarosite. 



260 



Subsurface Geologic Methods 



Ca rbonates 

Otgrees Ctnligrode 
SOO 700 



LOCAL I T Y 



Cgmb«flon0. Enjlontf I T 



lourium. Greece . I 1 -f- 



1 I I Smithtonilt 



II III Smiinson.ie 



Roibuty, Conn I I 



Oevonshne. Englond I j h 



1 I Sidenfe 



eurl.nqlon Mine. 



Lohe Co, Colo 



M.oik.U'Ol* I i *- 



Si E»'e«ne. S»irri» I H 



Cnsmouni, ^ronet 



utw aimoden.Coiif ' 1 r- 



Cumbefiond. £"91014 ' \- 




Ne. MtKCo I 1 1 1 1 1 1 1— >^ 1 \ 1/^ I CsiC'te 



' ■ I ' ' ' I ' I ' I ■ I ' I ' I ■ ' ' I 
100 300 500 700 900 MQO 

0*g'««» C*n<iarodt 



Figure 110. Thermal curves for a number of common rhombohedral carbonates. 



Subsurface Laboratory Methods 261 

position. Cuthbert and Rowland ^^ published thermal curves of several 
carbonate minerals. 

In these curves the carbonate peaks are low because of the admixture 
of inert material. The curves from figure 110 closely approximate the car- 
bonate specimens run by other workers.^° ^^ 

Artificial Mixtures 

Figures 111 to 118 show sets of thermal curves of predetermined mix- 
tures ground to 50-mesh.*^ Although theoretically the area under the 
curve should be proportional to the percentage of the mineral present, 
this does not strictly hold experimentally. It has been found, however, 
that for known mixtures the amplitude of the peak plotted against the 
percentage of the mineral present gives a smooth curve. Moreover, it 
has been found that this "calibration curve" is not particularly affected by 
the chemical nature of the other components. Using figures for artificial 
mixtures containing kaolinite to furnish data, the graph in figure 119 was 
prepared. The amplitude of the endothermic 605° C. peak for kaolinite 
is plotted against the percentage of kaolinite in the particular mixture. A 
different symbol is used for each mixture. The area within the two smooth 
curves indicates the possible error to be expected from a mixture of 
kaolinite with an unknown aggregation, as indicated by artificial mix- 
tures. Clay minerals that give such distinctive peaks as kaolinite may 
be quantitatively estimated with reasonable certainty for simple mixtures 
within ten or twenty percent. The variation may be due in part to minor 
differences in the heat conductivity of the foreign constituent. 

The necessary assumption to render valid the application of the cali- 
bration curve to an unknown mixture is that the clay minerals in the un- 
known must be in roughly the same physical and chemical condition as in 
the artificial mixtures. This is probably a good approximation in many 
cases, particularly in the case of hydrothermal clays formed in situ. Grim ^^ 
has already pointed out the need for great caution in making such an 
assumption for certain sedimentary-clay mixtures. 

Figure 111 shows a suite of kaolinite-goethite mixtures. The endo- 
thermic decomposition peaks for both minerals are shifted down in tem- 
perature with increasing percentage of the other mineral. This shift is to 

™ Cuthbert, F. L., and Rowland, R. A., Differential Thermal Analysis of Some Carbonate Minerals: 
Am. Mineralogist, vol. 32, p. Ill, 1947. 

™ Speil, Sidney, Berkelhamer, L. H., Pask, J. A., and Davies, Ben, op. cit. 
^ Faust, unpublished. (No reference given in original paper.) 

'- The samples used in these artificial mixtures were essentially uncontaminated materials from well- 
known localities and were checked both optically and by means of X-ray diffraction. The alunite sample 
was analyzed chemically by Ledoux and Company. 

Mineral Locality Mineral Locality 

Alunite. Santa Rita, New Mexico Quartz 

Jarosite Santa Maria mine, Sericite A.merican Canyon, Nevada 

Jelardena, Durango, Mexico Dickite _ Cusihuirachic, Mexico 

Kaolinite Dry Branch, Georgia GoethJte ._ „..Lake Superior 

Montmorillomte..Po'kville, Mississippi 
'^ Grim, R. E., Differential Thermal Curves of Prepared Mixtures of Clay Minerals: Am* Mineralogist. 
vol. 32, p. 493, 1947. 



262 Subsurface Geologic Methods 

be attributed to the conductance of the heat away from the particles in the 
endothermic reaction by the foreign inert neighbors. The 980° C. exo- 
thermic peak is not shifted appreciably. This is probably a result of the 
narrow temperature range of the reaction. Below a certain temperature, 
under these conditions of molecular structure, amorphous alumina will 
not change over to gamma alumina. At 980° C, however, crystallization 
occurs almost instantaneously. Hence, the mixture of 50-mesh inert mate- 
rial with 50-mesh kaolinite does not appreciably shift this peak. 

Figure 112 illustrates the effect of mixing quartz with kaolinite. The 
quartz curve is a straight line aside from a minor peak at the inversion 
point. The kaolinite curve is depressed by the admixture of quartz, but 
comparison with figure 105 indicates that otherwise there does not appear 
to be any substantial change. 

Figure 113 represents mixtures of sericite and kaolinite. Sericite 
shows little noticeable differential effect. On the other hand, even as little 
as ten percent kaolinite in a mixture with sericite may be detected. Since 
both minerals are common in zones of hydrothermal alteration, this fea- 
ture is of interest. 

Figure 114 contains curves of kaolinite and alunite, which represent 
a mixture of two thermally active minerals that may occur together in 
the same deposits. Both minerals yield sharp and distinctive thermal peaks. 

Figure 115 represents a sequence of thermal curves for alunite and 
jarosite where the samples are artificial mixtures. Both alunite (fig. 109) 
and kaolinite (fig. 105) are illustrated elsewhere. Where curves show 
such prominent peaks, mixtures may be studied with reasonable facility. 
A proportional decrease in the amplitude as well as a downward shift 
of peak temperatures occurs with an increase in foreign constituents. 

A common problem in the study of zones of argillic alteration con- 
cerns the estimation of the relative amounts of kaolinite and dickite present 
in a natural mixture. Figure 116 illustrates a series of artificial mixtures 
of the two minerals. 

Kaolinite-montmorillonite mixtures are illustrated in figure 117. Evi- 
dently the apparatus as normally employed is less sensitive for the de- 
tection of montmorillonite in a mixture than it is for distinguishing min- 
erals with higher temperatures and more distinctive thermal effects. 

Montmorillonite-sericite mixtures are indicated in figure 118. While 
montmorillonite would be detected in such mixtures, it seems likely that 
sericite would escape detection. It is evident that the effect of shifting 
the peaks with the percentage of impurity must be determined for each 
mineral properly to identify minerals in mixtures. The carbonates appear 
particularly sensitive to this effect. 

The curves above have been used effectively in the semiquantitative 
determination of the argillic constituents of an altered mineralized area. 
The application of the technique to this form of problem offers signifi- 



Subsurface Laboratory Methods 



263 



Kaolinite Goethite Mixtures 






10 90 |- 



29 75 |_ 



40 60 }- 



I H 



Degrees Centigrade 
500 700 



I 1 1 \ 



J \ L 




900 900 TOO 

Oagrtat C«nti«rotf« 



I I \ \ L 



Figure 111. Artificial mixtures of kaolinite and goethite arranged to illustrate possible 
interpretation of natural mixtures. 



264 



Subsurface Geologic Methods 



Kaolinite-Quartz Mixtures 



^ 



O ^ l OO . 30 

4^ <j* r ' I ' r 

100 1 I I 



300 



Degrees Centigrode 

T' I ' T ' I ' T ' I ' 'T 



10 90 



4-^^ 



25 75 



50 50 



75 25 



90 10 



H h 



I h 



I H 



100 I [. 



■^T" 





-A— 



K l^-i \ i- 



JL 



Jl 



I ■ I ■ I 



100 300 



500 700 900 

Degrees Centigrade 



HOC 



Figure 112. Curves indicating effect on kaolinite with quartz as an impurity. 



Subsurface Laboratory Methods 



265 



. Kaolinite-Sericite Mixtures 



t^ <^ iqo 300 

^* / I ' I ■ I ■ I ■ 



100 



10 90 



25 75 



40 60 



60 40 



75 25 



90 10 



100 



A 1- 



I \- 



I \ h 



H H 



Degr««s Centigrod« 

500 700 900 

I ■ I • I ' I ' I ' 



1100 



r-n 



■+-^ 



J — f- 



i^—i- 



->v- 




f-^H— I 



I I I I I ' I ' I ■ I ■ I , I . I , I ■ I 

'00 300 ioo 700 ' 900 ^UOO 

Degrees Contigrode 



Figure 113. Curves showing mixtures of almost inert sericite and active kaolinite. 



266 



Subsurface Geologic Methods 



Alunite-Kaolinite Mixtures 



^ !§■ Degrees Cenfigrode 

ov^ / r^ i ' f' I ' f' I 'T 



gf 1 1 , "f > 



T — I 1 T' I — I r 



100 



80 20 



60 40 



40 60 



25 75 



10 90 



100 



I H 



h ^ h 



h — ^ h 



i ^ 




■ I I I I I I I I \ 1 I L 



100 300 500 700 900 

Degrees Centigrode 



1100 



FiGUKE 114. Thermally active kaolinite and alunite in artificial mixtures. 



Subsurface Laboratory Methods 



267 



Alunite Jarosite Mixtures 




Degrees Centigrode 



Figure 115. Thermal curves of artificial jarosite mixtures. 



268 



Subsurface Geologic Methods 






KaOLINITE - DiCKITE MIXTURES 



Otfraat Cintigrod* 



HOO 



100 



10 90 



25 75 



40 60 



60 40 



79 25 



90 10 



100 O 



100 



^ 1 1 h 



■4 1 1 1-^ I 



I 1 1 1- 



H h 



I h 





I 1 -I 1 K^ I 





H ^ 



H 1 1 ^ 



I I 




+-^ I 




-\ 1- 



J 1 



1^ — I- 



< \- 













I — I I I I t I ■ I ■ I ' 1 ■ ' ■ ' ■ ' 



SCO aoo 700 

0«9r««« C«iititr«d« 



900 100 



Figure 116. Artificial mixtures of kaolinite and dickite showing variation 
in thermal curves. 



Subsurface Laboratory Methods 



269 



•4^ KaOLINITE- MONTMORILLONITE 

<? Mixtures 






Degrees Centigrade 



^ «^ lOO 300 500 700 900 1100 

o^ ^^ T ■ I ' I ' I ' I ' I ' I ' I ' I ■ I ' I 



100 



10 90 



2S 75 



40 60 



60 40 



75 25 



90 10 



100 



H ^ 



^ — I 





r — I 



' I i I ■ I ' 



I I I 1_J 1 I I L. 



100 300 500 700 900 

Degrees Centigrade 



Figure 117. Thermal curves of artificial kaolinite-montmorillonite mixtures with a 
range from to 100 percent. 



270 



Subsurface Geologic Methods 



MONTMORILLONITE -SeRICITE MIXTURES 






.300. 



Otgreas Centigrods 



fo ■ I ■ 5^0 , I . 7y> , ^ , gyt 



liPO 






90 10 



-\ 1- 



75 25 



■J 1 }— 



60 40 



40 60 



H h 



25 75 



10 90 



100 



t h 



I L 




--}- (^ I 



00 300 500 roS 5ocr 

Otgrees Centigrodt 



■nic 



100 



Figure 118. Artificial mixtures of almost inert sericite with montmorillonite. 



Subsurface Laboratory Methods 



271 



cant possibilities in mapping alteration zones associated with mineral 
deposits in studies of the type reviewed by Kerr.^^ 

The thermal curves thus far obtained in these studies are for the most 
part consistent with curves recorded in the literature, allowing for the 



100 



80 



60 - 



% KAOLINITE 
IN MIXTURE 



40 



20 - 







1 


1 — 


/ * / 


. 








/ / 










••••OO / 


- 






' aMCK) 


/ 


■ 




/ ' 




• 


- 




/ •••<>o / 




EXPLANATION 




/•••<)0 ^ 




o 


Koolinil* - Godhilt 


■ / 


/ • y^ 




• 
• 


KooMflil* - Strieit* 
Koollnitt ■ Dickitt 
Koolinilt • Ouarti 
Koollnit* ■ Alunlt* 


/^ 






• 


Keolinilt • MonlmorltloniK. 




10 


20 


30 


4 SC 



100 



- 80 



40 



20 



PEAK AMPLITUDE (arbitrary units) 

Figure 119. Graph showing variation in position and amplitude of the 605° C. peak 
of kaolinite in various artificial mixtures. 



variation in heating rates. The temperatures at which peaks occur have 
been agreed upon by various observers with different types of apparatus, 
if the heating rates, the thermocouples, and the size of sample are con- 
stant. The amplitude of the peaks for any given concentration of active 
ingredient is a function of the sensitivity of the individual apparatus. 



'Kerr, P. F., Alteration Studies: Am. Mineralogist, vol. 32, p. 158, 1947. 



272 Subsurface Geologic Methods 

WATER ANALYSIS 

(CHARACTERISTICS OF OIL-FIELD WATERS OF THE 

ROCKY MOUNTAIN REGION) 

JAMES G. CRAWFORD 

The identification and correlation of waters found in drilling and 
producing oil and gas wells with definite lithologic units have been ap- 
plications of water analysis of great direct value to the oil-production 
industry. Correlations are made upon the premises that the composition 
of the water from a given zone is constant, or nearly constant, throughout 
the economic life of an oil or gas field and that the water contained in 
each producing zone has diagnostic characteristics by which it can be 
distinguished from every other water above or below that zone in the im- 
mediate vicinity. 

Structural or stratigraphic traps capable of acting as reservoirs for 
hydrocarbons also act as reservoirs for water. Thus, waters associated 
with oil or gas are relatively stagnant and usually present an entirely dif- 
ferent set of properties from surface waters or circulating ground waters 
in the immediate vicinity. Although waters are identified and correlated 
by comparison with known samples in the immediate area, it has been 
found that regional correlations, although somewhat inexact, are possible. 

The variation in composition and concentration of formation waters 
sampled over a structure should be recognized. Water tables in the Rocky 
Mountain region are as a rule tilted, and the water yielded on the high 
side of the structure, although of the same general characteristics, often 
differs from that of the low side. These variations may be appreciable or 
may not be particularly noticeable, depending upon the field and area, 
but analyses must be interpreted with these possible variations in mind. 

It is the purpose of this section to discuss briefly the regional similar- 
ities and differences of oil-field waters in the Rocky Mountain area, with 
particular emphasis on correlation with definite lithologic units. 

The writer is indebted to the United States Geological Survey for 
many of the analyses from which correlations could be made; to the oil 
companies of the Rocky Mountain region for the many analyses furnished ; 
to H. E. Summerford for his valuable assistance in the preparation of 
the geologic information; to J. A. Waatti for preparing the illustrations; 
and to R. M. Larsen for general and specific criticism. 

Classification of Waters 

The Palmer ^^ system of water classification emphasizes important dif- 
ferences between waters in geochemical relationship and has been used 
throughout this paper in the discussion of types of water. The Palmer 
system groups those radicles that are either chemically similar or geo- 
logically associated: Sodium and potassium are grouped as alkalies; cal- 

^ Palmer, Chase, The Geochemical Inter pr elation of Water Analyses: U. S. Geol. Survey Bull. 
479, 1911. 



Subsurface Laboratory Methods 273 

cium and magnesium are grouped as alkaline earths; sulphates, chlorides, 
and nitrates are grouped as strong acids; and carbonates, bicarbonates, 
and sulphides are grouped as weak acids. Thus, according to the reacting 
value of these four groups, natural waters can be classified into four 
types, i.e., primary saline, secondary saline, primary alkaline, and sec- 
ondary alkaline. 

An excess of strong acids over weak acids causes salinity. It should 
be noted that salinity can be due to either the sulphate or chloride radicle 
or to both. The alkalies in connection with the strong acids cause primary 
salinity, and an excess of strong acids with an equal value of alkaline 
earths induces secondary salinity. 

An excess of alkalies over the strong acids with an equal value of 
the weak acids makes up primary alkalinity, and an excess of weak acids 
combined with aa equal value of alkaline earths produces secondary 
alkalinity. 

Secondary salinity and primary alkalinity are incompatible; thus, 
each natural water will have two or three of the above-mentioned proper- 
ties but never all four. 

Primary salinity is common to all waters, and a primary saline water 
is essentially a solution of sodium and potassium sulphates and chlorides. 
A primary alkaline water consists principally of sodium and potassium 
carbonates and bicarbonates. Calcium and magnesium sulphates and 
chlorides predominate in a secondary saline water, and the water is per- 
manently hard. Temporary hardness is present in a secondary alkaline 
water consisting principally of calcium and magnesium bicarbonates. 

Surface Waters 

Surface waters in the Rocky Mountain region range from the dilute, 
soft, alkaline waters of igneous terrain to moderately concentrated, hard, 
rine beds. The usual mountain water derived from melting snow is soft, 
alkaline, and dilute, but, after it has traversed marine sediments, calcium 
and magnesium sulphates dominate the chemical system, and the water 
often takes on a load of salts that makes it unfit to drink. 

The North Platte River, for example, rises in a network of mountain 
streams in North Park, Colorado, and drains the southeastern quarter of 
Wyoming. Near its source it is a primary alkaline type, but by the time 
it reaches the Pathfinder reservoir it has been changed to a secondary sa- 
line type. The Popo Agie, though dilute, is secondary saline near Lander, 
Wyoming, whereas Castle Creek, Teapot Creek, and Salt Creek waters are 
undrinkable because of the alkaline earth salts leached from the Steele 
shale, as are the waters of many other smaller streams of Wyoming whose 
drainage does not embrace the higher mountain areas. 

The influence of surface waters upon formation waters encountered 
in drilling wells c^n be observed in a number of instances in Wyoming. 
The most striking example is the Shannon sandstone along the western 



274 Subsurface Geologic Methods 

edge of the Powder River Basin, where the influence of primary alkaline 
waters from the Big Horn Mountains is indicated at Billy Creek, and the 
influence of secondary saline surface waters can be traced in the Salt 
Creek area. 

In general, the presence of a secondary saline water in Cretaceous 
and younger sands of Wyoming indicate surface-water infiltration or 
contamination. Sulphate is absent or negligible in Cretaceous waters, and 
the presence of this radicle almost invariably indicates drilling-water con- 
tamination, particularly as sulphate is the dominant negative ion in most 
surface waters. The presence of sulphate in persistent and notable quanti- 
ties usually is not encountered below the ground-water zone until Juras- 
sic beds are reached. Below the Jurassic, however, sulphate is usually the 
principal negative ion. 

Tertiary 

Tertiary beds of unconsolidated variegated shales and sandstones 
cover most of the plain and basin areas of the Rocky Mountain region with 
thicknesses up to more than 30,000 feet. Most of these sands are not pro- 
ductive of commercial oil or gas. Small amounts of oil and gas have been 
produced from the White River formations of Oligocene age at Shawnee, 
Douglas, and Brenning Basin in central-eastern Wyoming, and oil and 
gas are being produced commercially from lenticular sand bodies in the 
Wasatch formation of Eocene age at Hiawatha and Powder Wash, Colo- 
rado, and La Barge, Wyoming. 

With the exception of the above-mentioned producing fields, the 
water analyses available from Tertiary sands were sampled for drilling 
use and do not reflect the stagnant conditions associated with oil-field 
waters. 

Green River Formation 

An exceptional type of water has been encountered in a few wells 
drilled into the lacustrine Green River formation of Eocene age. The for- 
mation consists of sandstone, marl, limestone, and sandy shale, with thin 
beds of oil shale, halite, glauberite, and trona. These salts have influenced 
the composition of the ground water in this area with the result that so- 
dium carbonate occupies 60 percent of the dissolved salt content of the 
water, sodium chloride 37 percent, and sodium sulphate 3 percent. Total 
solids range from 40,000 to 80,000 parts per million, and the water has 
been used for the production of crude soda ash. 

Wasatch Formation 

The Wasatch formation at Hiawatha and Powder Wash, Colorado, 
consists of more than 5,000 feet of shale containing irregular and lenticu- 
lar fluviatile and lacustrine porous sands. Oil and gas production at Hia- 
watha comes from three lenticular oil sands between depths of 2,032 and 



Subsurface Laboratory Methods 275 

2,512 feet, and at Powder Wash from zones logged in one well at 3,087 to 
3,113 feet and 5,014 to 5,023 feet. The formation contains numerous 
water-bearing lenticular sand bodies in addition to the oil- and gas-pro- 
ducing zones. 

The La Barge field in western Wyoming produces oil from the Wa- 
satch formation. The producing zone, at depths of 650 to 1,100 feet, 
consists of two to three divisions, the upper a persistent sandstone aver- 
aging about twenty feet in thickness, and the lower a series of sandstones 
separated by interfingering shale beds that vary greatly in thickness. 

Wasatch waters are, on the whole, saline, the salinity being due al- 
most entirely to the chloride ion. Concentrations range in total solids 
from 1,500 to as much as 32,000 parts per million at Hiawatha and Powder 
Wash, and from 3,000 to 12,000 parts per m-illion at La Barge. The more 
concentrated and saline water at La Barge occurs in the lower and less- 
continuous zones. Secondary characteristics are relatively low, though 
variable, in the Colorado Wasatch waters and are negligible in the La 
Barge waters. 

Lenticularity and lack of continuity are indicated by the erratic and 
variable nature of the Hiawatha and Powder Wash waters. The uniformity 
of the La Barge analyses points to the more continuous nature of the 
sandstones. Representative Wasatch waters are tabulated in table 9. 

Upper Cretaceous 

Upper Cretaceous beds have yielded a major portion of the oil and 
gas production in the Rocky Mountain region and, although overshadowed 
now by pre-Triassic exploration, do and will continue to hold an im- 
portant place in Rocky Mountain oil production. A basal sandstone of the 
Mesaverde formation has produced some oil at Simpson Ridge, Wyoming, 
and is producing oil now at West Poison Spider, Wyoming. The principal 
Upper Cretaceous oil-producing zones in Wyoming are the Wall Creek 
and equivalent sandstone beds of the Frontier formation and the Muddy 
(Newcastle) sandstone member of the Thermopolis shale. 

Oil production from Upper Cretaceous sands in Montana has been 
limited to a few areas, the most important being Cat Creek, but these 
sands are important gas producers throughout the Great Plains region of 
the state. 

There has been some oil and gas production from Upper Cretaceous 
beds in Colorado, principally from fractured sandy zones in the Mancos 
shale, but Lower Cretaceous and older beds are the more prolific horizons. 

Montana Group 

Montana-group waters are important in oil-field operations in the 
state of Montana more for identification of intrusive water than for any 
other purpose. With the exception of gas-producing fields, surface-water 



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Subsurface Laboratory Methods 277 

characteristic? usually predominate, and the available reanalyses are so 
scattered that generalization is difficult. 

The Two Medicine and Eagle sandstones at Cut Bank yield waters of 
dilute to moderate concentration — from 800 to 8,000 parts per million 
total solids — consisting principally of sodium sulphate and sodium bicar- 
bonate. Calcium and magnesium are absent or are present in small quanti- 
ties only, and chloride seldom exceeds 100 parts per million. The Eagle 
waters are more dilute than the Two Medicine waters and average about 
1,200 parts per million total solids. Both waters show ground-water 
characteristics and are usually shut off with a single string of casing. 

In contrast, the Judith River waters encountered in the gas-producing 
Cedar Creek anticline are saline with concentrations ranging from 8,000 
to 15,000 parts per million total solids; chloride varies from 4,000 to 
10,000 parts per million; sulphate is absent or is present only in neg- 
ligible quantities; bicarbonate is low, averaging about 200 parts per mil- 
lion; and calcium and magnesium are present in relatively small but defi- 
nite quantities. These waters are associated with natural gas, and their 
higher-than-average concentrations are ascribed to the evaporative effect 
of the gas. It has been found that as a general rule throughout the Rocky 
Mountain region waters associated with gas are more concentrated than 
waters associated with oil. 

The extreme difference in composition between the surface-water 
type at Cut Bank and the chloride-saline type in gas-producing fields is 
shown in the ionic statements in table 9. 

Colorado Group 

The Colorado group in Montana includes, besides dark fissile shale, 
sandy and sandstone members that yield oil or gas at several localities. 
In Wyoming the Colorado group includes the Shannon sandstone, the 
important oil-producing Frontier formation, and the Muddy (Newcastle) 
sandstone. 

The Blackleaf sandy member of the Colorado shale yields gas with 
showings of oil in a number of Montana fields. The waters encountered 
in this member at Border-Red Coulee, Cut Bank, Kevin-Sunburst, and 
Bowdoin are essentially solutions of sodium chloride ranging from 6,000 
to 16,000 parts per million total solids. Sulphate is negligible in these 
waters, and secondary characteristics are low but persistent, ranging from 
50 to 200 parts per million calcium and a trace to 100 parts per million 
magnesium. Primary salinity averages ninety percent, primary alkalinity 
six percent, and secondary alkalinity four percent of the chemical system. 
It is interesting to note that these waters resemble Judith River water as 
found in the Cedar Creek anticline; thus the concentrations and salinity 
can be ascribed to their association with gas. 

First Cat Creek sandstone waters at Cat Creek are moderately dilute 
and balanced, the alkalinity and salinity each averaging about fifty per- 



278 Subsurface Geologic Methods 

cent of the system. The concentrations vary from about 1,000 to 2,000 
parts per million total solids, and the alkaline earths and sulphate are 
absent or negligible. The Cat Creek field is one of the areas in the Rocky 
Mountain region in which a dilute to moderately dilute water is found 
associated with commercial oil production. 

Three typical Colorado-group waters are tabulated in table 9. 

Shannon Sandstone 

Although the Shannon sandstone has been tested in many wells, the 
only fields now producing from it are Cole Creek and Big Muddy, Wyo- 
ming. At Cole Creek the Shannon sandstone is found at a depth of approx- 
imately 4,500 feet. A concentrated, saline water averaging about 18,000 
parts per million total solids is produced with the oil from edge wells. The 
water averages about six percent secondary alkalinity, and sulphate is 
present in quantities of 50 to 500 parts per million. This water, although 
more concentrated and with little higher alkalinity, resembles equivalent 
water at Big Muddy. 

The top of the Shannon sandstone in the Big Muddy field occurs at 
a depth of about 900 feet and consists of about thirty feet of alternating 
lenses of buff to gray sandstone and sandy shale carrying both oil and 
water. The water is a solution of sodium chloride varying from 9,000 to 
15,000 parts per million total solids, with about three percent secondary 
characteristics; bicarbonate is relatively low, ranging from 450 to 700 
parts per million. 

It is interesting to note the variation of the Shannon waters along 
the western edge of the Powder River Basin. The Shannon sandstone in 
the Billy Creek gas field yielded a balanced water, i.e., a water in which 
alkalinity and salinity occupy about fifty percent each in the chemical 
system, with no secondary characteristics or alkaline earths; concentra- 
tions ranged from 2,000 to 3,000 parts per million total solids. The sand- 
stone in this field is 900 to 1,300 feet below the surface and is fed by 
fresh, sulphate-free alkaline water from the nearby Big Horn Mountains. 

The influence of secondary-saline surface water can be seen in the 
Salt Creek area, where the Shannon sandstone forms an escarpment on 
the east and west sides of the Salt Creek uplift. Here the feed is the 
gypsum-impregnated waters of Castle Creek, Teapot Creek, and Salt 
Creek, and the Shannon formation waters encountered during drilling 
were practically identical to these surface waters. 

Saline waters of relatively high concentration are associated with 
oil production from the Shannon sandstone at Cole Creek and Big Muddy, 
and it is concluded that they have not been influenced to any extent by 
surface-water infiltration. 

Thus, it is possible in this one area, trending northwest-southeast 
along the western edge of the Powder River Basin, to observe the effects 
of surface-water infiltration of two different types upon the connate water 



Subsurface Laboratory Methods 279 

originally in the sand, as typified by Cole Creek and Big Muddy waters. 
Representative analyses of all these waters are given in table 9. 

Frontier Formation, 

The Frontier formation has been one of the most productive oil hori- 
zons in the Rocky Mountain region. It is overshadowed now by pre- 
Triassic production but nevertheless still is an actual and potential oil 
producer of large capacity. Where it produces oil the formation ranges 
in thickness from 370 to 1,200 feet and contains from two to nine beds of 
sandstone. Where the several Wall Creek sands can be separated, the 
designation of "First Wall Creek," "Second Wall Creek," et cetera, are 
given them; where the separate sands cannot be identified, water samples 
are designated simply as "Frontier." Frontier sands where identifiable 
in the Big Horn Basin are termed "Torchlight" and "Peay." 

Frontier waters are for the most part solutions of sodium chloride 
and sodium bicarbonate in varying proportions. Calcium and magnesium 
occur in small amounts in some waters and are absent in others; in no 
case is there sufficient calcium or magnesium to give secondary salinity 
to the water. Sulphate is absent or is present in minor quantities only. 
Concentrations are quite variable, ranging from about 1,200 parts per 
million to 50,000 parts per million total solids. 

Representative analyses of a number of the more important Frontier 
waters of Wyoming are tabulated in table 10. The variation in concentra- 
tion of these waters is due, it is believed, to several causes, among them 
and most important being the lenticularity of the sands and their perme- 
ability development. It is believed that the more dilute and alkaline 
waters have been modified considerably by meteoric waters, whereas the 
more concentrated, saline waters are assumed to be connate without any 
substantial modification by meteoric waters since accumulation. 

It is interesting to note from table 10 that the highest concentrated 
waters are associated with gas in the Baxter Basin fields. These, together 
with the Montana-group waters cited above, tend to support the contention 
of Mills and Wells ^^ that water can become concentrated at depth by the 
agency of moving and expanding gas. 

Muddy (Newcastle) Sand 

Oil production from the Muddy sand had been small and scattered 
until the devlopment of the Mush Creek and Skull Creek areas along the 
eastern edge of the Powder River Basin. Here a twelve- to twenty-foot 
section of medium- to fine-grained, slightly tripolitic sand with coal inter- 
calations interbedded with shales, locally called the "Newcastle sand- 
stone," yields commercial oil production. 

The Newcastle water at Mush Creek is a primary-saline water rang- 
ing from 10,000 to 15,000 parts per million total solids; secondary char- 

^^ Mills, R. Van A., and Wells, R. C, The Evaporation and Concentration of Waters Associated with 
Petroleum and Natural Gas: U. S. Geol. Survey Bull. 693, 1919. 



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Worland _.. 



Subsurface Laboratory Methods 281 

acteristics and sulphate are negligible; chloride ranges from 3,000 to 
7,000 parts per million depending upon the concentration; bicarbonate, 
though, is more erratic and varies from a low of 1,300 to a high of 6,000 
parts per million. 

In contrast to Mush Creek, the Newcastle-sandstone waters at Skull 
Creek are alkaline and vary from 6,000 to 8,000 parts per million total 
solids. Secondary characteristics and sulphate are negligible, chloride 
ranges from 800 to 2,000 parts per million, and bicarbonate from 4,000 
to 6,000 parts per million. This, the author believes, is evidence that the 
Skull Creek waters have been modified by meteoric water to a greater 
extent than the Mush Creek waters. 

Lower Cretaceous 

Lower Cretaceous beds include the Cloverly formation of Wyoming 
and Colorado, the Greybull sandstone and Pryor conglomerate of south- 
central Montana and the Big Horn Basin of Wyoming, and the Kootenai 
formation of north and north-central Montana. These beds produce oil 
at various localities in Wyoming and Colorado, and the Kootenai is one 
of the principal oil- and gas-producing formations of Montana. 

In general, Dakota(?) waters of Colorado are relatively dilute, soft, 
and alkaline, with concentrations ranging from 700 to 3,000 parts per 
million total solids and with an average of about 1,500 parts per million 
total solids. Wyoming Dakota(?) waters are quite variable; where as- 
sociated with oil or gas, concentrations as high as 20,000 parts per million 
total solids are encountered, but where not associated with hydrocarbon 
accumulation dilute waters are the rule. Kootenai waters range from about 
1,000 parts per million at Cat Creek to as high as 15,000 parts per million 
total solids at Cut Bank and on the average seem to be more concentrated 
than equivalent waters in Colorado and Wyoming. 

Dakota(?) and Kootenai waters as a rule are more alkaline than 
Upper Cretaceous waters previously discussed. Alkalinity is the impor- 
tant distinguishing feature between Colorado and Kootenai waters in 
Montana, the bicarbonate content of Kootenai waters being appreciably 
higher. Exceptions to this rule in Wyoming, however, are numerous, the 
North Baxter Basin gas field being an example of high salinity and low 
alkalinity; the Beaver Creek gas field also violates the general rule, as 
do Salt Creek and Church Buttes. 

The Dakota(?) and Lakota sands more often than any other forma- 
tion yield potable water in Wyoming. This is particularly noticeable 
along the eastern edge of the Powder River Basin and in the Poison Spider 
area of central Wyoming, where the sands are not deeply buried and crop 
out on nearby uplifts. 

Table 11 lists a number of the more important Lower Cretaceous 
waters of the Rocky Mountain region. 





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Subsurface Laboratory Methods 283 

Jurassic 

Marine Jurassic beds include the important Sundance sandstone of 
Colorado and central and eastern Wyoming, the Nugget of western Wyo- 
ming and western Colorado, and the Ellis formation of Montana. The 
Sundance is an important oil producer in Colorado at lies, Moffat, and 
Wilson Creek and in Wyoming at Salt Creek, Lance Creek, Big Medi- 
cine Bow, Rock Creek, and other localities. Oil production at Kevin- 
Sunburst, Montana, comes from the zone at the contact of the Ellis for- 
mation and Madison limestone, much of which is from the reworked 
Madison limestone. 

The Morrison formation overlies the Sundance in Colorado and Wyo- 
ming and represents the transition zone from Lower Cretaceous to the 
marine Jurassic. Its waters are included in this section. The Morrison 
produces oil at Wilson Creek, Colorado, and in extent and productivity 
outranks the Sundance. Scattered showings of oil and gas have been en- 
countered elsewhere in the Morrison formation. 

Morrison waters in Colorado range from soft, alkaline types to saline 
waters containing appreciable hardness. They vary in concentration from 
about 3,000 to 15,000 parts per million total solids and usually contain 
appreciable amounts of sulphate. Most of the Sundance waters of Colo- 
rado do not contain sulphate — by which they can be distinguished from 
Morrison waters — and the alkaline and moderately dilute waters are 
usually soft. The saline waters generally contain appreciable hardness 
and in many respects resemble Morrison waters. 

The Ellis waters of Montana are quite uniform over the northern 
portion of the plains and consist principally of sodium chloride and 
sodium bicarbonate with minor but persistent secondary characteristics. 
These waters almost invariably contain hydrogen, sulphide, and, with the 
exception of Cosmos-Vanalto waters at Border-Red Coulee, are the young- 
est waters of the state to carry hydrogen sulphides. They average about 
3,500 parts per million total solids. 

The Sundance waters of Wyoming are rather variable in concentration 
and composition. They range from a low of about 1,200 parts per million 
total solids at Big Medicine Bow to as high as 40,000 parts per million at 
Steamboat Butte, where the water takes on evaporite characteristics similar 
to Triassic waters. The Sundance is the youngest formation in Wyoming 
in which secondary salinity becomes an appreciable and persistent part of 
the chemical system, yet secondary salinity is present only in a few fields 
such as Alkali Butte, the third sand at Salt Creek, the basal sand at Lance 
Creek, and other scattered localities. It is believed that secondary salinity 
is persistently present in Sundance waters in those beds in which limestone 
predominates; where sandstone predominates, the water is of the same 
general character — relatively soft, with little or no sulphate — as Cretaceous 
waters. Thus, it is inferred that the characteristics and composition of for- 







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Subsurface Laboratory Methods 285 

mation waters depend to some extent at least upon the petrography of the 
rocks. 

Representative Morrison and Sundance waters are tabulated in table 
12. 

Triassic 

Small amounts of oil have been produced from the basal part of the 
Moenkopi formation of Triassic age in Utah, and oil production has 
come from stray sandstone beds in the Chugwater formation of Permian 
and Triassic age at Grass Creek and Hamilton Dome, Wyoming, but for 
the most part Triassic beds have been dry. Water analyses of Triassic 
age have been few and scattered, but the Chugwater waters of Wyoming 
that have been sampled are highly concentrated solutions of sodium sul- 
phate and sodium chloride averaging from 40,000 to 60,000 parts per 
million total solids; alkalinity is negligible in these waters, the bicarbonate 
usually averaging about 200 parts per million. 

Triassic beds are the dividing line between the post-Triassic primary 
waters and the pre-Triassic secondary waters of the Rocky Mountain 
region. Most of the waters in zones younger than Triassic contain few, 
if any, secondary characteristics; in contrast, secondary characteristics 
dominate the waters of formations older than Triassic. 

Permian 

Permian oil production in the Rocky Mountain region is limited prin- 
cipally to the Embar formation in Wyoming, consisting of porous dolomite, 
limestone, and chert. Much of the exploration work in the Rocky Moun- 
tain region since 1940 has been in the Embar and older formations, and 
these now outrank the post-Triassic beds in productivity. 

Embar waters range from 1,800 parts per million total solids at 
Dallas Dome to 38,000 parts per million total solids at Neiber Dome, but 
the average concentration ranges from 4,000 to 7,000 parts per million 
total solids. With but few exceptions these waters are solutions of sodium, 
calcium, and magnesium sulphates in varying proportions. Sulphate 
salinity exceeds chloride salinity, and the bicarbonate ion usually is rela- 
tively low; most of these waters carry hydrogen sulphide. 

The Embar water at Neiber Dome is very unusual in that there is a 
large amount of alkalinity present as the bicarbonate radicle. This is 
decidedly out of line in a post-Triassic water, but there have not been sufl&- 
cient samples from this structure for postulations concerning its source. 

Typical Embar waters of Wyoming are tabulated in table 13. 

Pennsylvanian 

The marine Pennsylvanian beds include the important oil-producing 
Tensleep sandstone of the Rocky Mountain region, the Amsden forma- 
tion, and their equivalent of eastern Wyoming, the Minnelusa formation. 



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Subsurface Laboratory Methods 289 

These beds also include the Weber sandstone of Colorado and Utah and 
the Quadrant formation of Montana. 

The greater part of the oil production of the Rocky Mountain 
region now comes from Pennsylvanian beds. Larger and more productive 
fields producing from Pennsylvanian beds include Lance Creek (Minne- 
lusa), Elk Basin (Tensleep), Salt Creek (Tensleep), Steamboat Butte 
(Tensleep), Wertz (Tensleep), Rangely (Weber), and Lost Soldier (Ten- 
sleep) . The Quadrant formation of Montana has yielded a negligible 
amount of oil in central Montana but for the most part produces only 
copious quantities of water. 

In general, Pennsylvanian waters are saline, with the salinity being 
due principally to the sulphate ion. Like the Permian waters previously 
discussed, the Pennsylvanian waters are marked by persistent and appre- 
ciable secondary characteristics; bicarbonate is usually low. Hydrogen 
sulphide is commonly present but usually not in any great quantity. 

The Weber water in Colorado and Utah is a chloride-saline type 
with appreciable quantities of sulphate. At Rangely a brine varying from 
about 100,000 parts per million to 150,000 parts per million total solids 
is associated with oil, and this is the highest-concentrated oil-field water 
from a producing field in the Rocky Mountain region. The Rangely brine 
is principally sodium chloride, the chloride ion ranging from 60,000 to 
100,000 parts per million and the sodium from 35,000 to 60,000 parts per 
million; calcium averages 5,000 parts per million and magnesium about 
650 parts per million. 

The Quadrant waters of Montana are typical Pennsylvanian waters. 
They average about 3,000 parts per million total solids and consist prin- 
cipally of the sulphates of sodium, calcium, and magnesium. Secondary 
salinity occupies from forty to seventy percent of the chemical system, 
and traces of hydrogen sulphide are usually found in the fresh water. 
Artesian flows, usually hot, of 10,000 to 125,000 barrels a day are en- 
countered in the Quadrant formation in the Montana fields tabulated in 
table 14, and these waters are the youngest encountered in north-central 
and central Montana that consistently contain large quantities of sulphate 
and the alkaline earths. 

The area in Montana embracing the new fields of Big Wall, Melstone, 
and Ragged Point is now controversial as to the age of the producing hori- 
zon. Water analyses seem to indicate that the three fields are producing 
from lithologically identical units, but present opinion places Big Wall 
and Melstone production as Amsden and Ragged Point as Kibby of 
Mississippian age. The author has placed all analyses in the Pennsyl- 
vanian table as Amsden(?) , but further study of the formation may change 
this grouping. 

Tensleep and Minnelusa waters in Wyoming vary from dilute to mod- 
erately concentrated (from a low of 200 parts per million at Derby Dorne 
to 13,000 parts per million at Quealy Dome) but on the whole average 



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292 Subsurface Geologic Methods 

from 3,000 to 4,000 parts per million total solids. Secondary salinity often 
dominates the chemical system, and even when it does not dominate it is 
appreciable. Like Embar waters, sulphate is the principal negative radicle, 
and bicarbonate is usually low. 

The dilute Tensleep waters in table 14, Dallas, Derby, and Lander, 
are artesian even though associated with oil. The Popo Agie River is ap- 
parently the source of these waters, and it is surmised that the Tensleep 
sandstone in these fields is subjected to an active, vigorous natural water 
drive; it is estimated that about four barrels of water are produced for 
each barrel of oil. Dilute water also is associated with oil at Black Moun- 
tain, Wyoming. 

MiSSISSIPPIAN 

The important Mississippian oil-producing zone is the Madison lime- 
stone and its Black Hills equivalent, the Pahasapa. It was the oldest sedi- 
mentary formation in the Rocky Mountain region to produce oil until the 
recent discovery of oil in a Cambrian sandstone at Lost Soldier, Wyoming. 
The Madison limestone yields oil in Montana and in the Lost Soldier- 
Wertz area and the Big Horn Basin of Wyoming and has yielded showings 
of oil in other parts of the region. (See table 15.) 

Madison waters of Montana are somewhat variable. Chloride salinity 
dominates in the fields around Sweetgrass arch and Sweetgrass Hills, and 
sulphate salinity is predominant in the central and north-central fields. 
The average concentration is about 5,000 parts per million total solids, and 
calcium and magnesium are present to some extent in all Madison waters, 
although less pronounced in the chloride-saline type. 

Madison waters from Wyoming fields, with the exception of one or 
two extremely dilute waters, seem to be more uniform than equivalent 
Montana waters. As a rule there is more secondary salinity in Madison 
waters than in Embar or Tensleep waters, and Madison waters usually are 
more dilute. The average concentration of Madison waters is about 2,500 
parts per million; sulphate usually dominates the negative ions, although 
there are a few waters, such as Torchlight, in which chloride dominates. 
Bicarbonate usually is low, averaging less than 500 parts per million, and 
in this respect the Madison waters resemble the average Tensleep water. 

Although there is a marked similarity between pre-Triassic waters, it 
is not difiicult usually to distinguish among Embar, Tensleep, and Madi- 
son waters in the same field or area. There are sufficient differences in con- 
centration, alkalinity, or sulphate-chloride ratio to correlate each water 
with its lithologic unit and make identification relatively easy. 

Devonian and Older 

Showings of oil have been found in pre-Cambrian crystallines, Cam- 
brian strata, Ordovician strata, and Devonian strata at widely separated 
localities in the Rocky Mountain region. Some geologists believe that the 



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294 Subsurface Geologic Methods 

Bighorn dolomite of Upper Ordovician age yields some of the deepest oil 
produced at Garland, Wyoming. But for many years, until 1948, the Madi- 
son limestone of Mississippian age was the oldest proved commercial oil 
zone in the Rocky Mountain region. In 1948 a Cambrian sand was proved 
for commercial oil production at Lost Soldier and Wertz, Wyoming, and 
the search for oil in pre-Mississippian beds was intensified. 

Several wells, particularly in Montana, have penetrated Ordovician 
and older beds, and a few analyses are available of these older waters, 
although the number is insufi&cient to warrant generalizing. The Ordovician 
waters analyzed appear to be of the same general character as Madison 
waters in the same well, whereas Devonian waters so far analyzed appear 
to be a more concentrated, more saline type, marked in particular by high 
calcium content. One typical Devonian water has a concentration of total 
solids of 19,000 parts per million and a calcium content of 1,096 parts per 
million; another Devonian water has a concentration of total solids of 
11,000 parts per million and a calcium content of 1,027 parts per million. 
The chloride-sulphate ratio was 4:1 in the first water and 2:1 in the 
second. 

It is believed, however, that these older waters as a whole will not 
be substantially different from other pre-Triassic waters, and that lime- 
stone characteristics will be the rule and not the exception. 

Conclusion 

The value of analyses as a means of identification of intrusive waters 
in well bores has been well established in the Rocky Mountain region. 
This is the primary purpose of water analyses, and the application of such 
data to engineering and geologic problems and theory is secondary. It is 
established definitely that there are sufficient differences in concentration 
and composition to correlate a water with its reservoir zone so that it can 
be differentiated from all other waters above or below that zone in a par- 
ticular well or area. 

The generally dilute nature of the oil-field waters of the Rocky Moun- 
ain region indicates extensive modification and dilution by meteoric 
ivaters. Some of the waters encountered seem to indicate little change 
since deposition, so that it is concluded that for these waters modification 
and dilution occurred before deposition; other waters seem to indicate 
extensive modification since deposition. In any respect, the brines com- 
monly associated with oil in other provinces of the world do not occur in 
the Rocky Mountain region. 

It is apparent that the oil-field waters in the Rocky Mountain region 
have been influenced by the petrography of the rocks in which they occur. 
Secondary characteristics and sulphate are at a minimum in waters of 
sandstone reservoirs of Cretaceous age or younger, whereas secondary 
characteristics and sulphate are prominent in the limestones and limy 
formations of pre-Triassic age. 



Subsurface Laboratory Methods 



295 



CORE ANALYSIS— PREDICTING WELL BEHAVIOR 
JOHN G. CARAN 

Core analysis has proved a valuable aid in the successful exploration, 
exploitation, and evaluation of gas and oil reserves. The basic core data 
make possible the location of fluid contacts and the prediction of the type 
of production to be expected. 

It is readily admitted that core analysis is but one of the useful tools 



PERMEABILITY 


CO RE- LOG 

(Note: Ser tiau page footnotfi jof ixpUnation of ahbripi 

-MiLLIOAHCYS O— O 

50 600 250 

1 1 1 1 1 


tiont or symbols in pdrenthties.) 

PRODUCTION INDEX O— 


1 1 1 




POROSITY -PEB 

50 40 3 


CENT • • 


RESIDUAL OIL-% PORE space •- — • 

20 40 60 80 100 






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Figure 122. Core-log of well A. 

available to the modern completion engineer. The use of core data, to- 
gether with a knowledge of the structural position of the well, the study 
of electric logs, and comprehensive drill-stem testing, minimizes the possi- 
bility of completing a dry hole or missing a productive formation. 



296 



Subsurface Geologic Methods 



No attempt has been made to discuss in detail all of the techniques 
employed by the various research and commercial core-testing laboratories. 
Analysis procedures vary for the formations being tested, the depth and 
pressure of the reservoir, and the type of core sample. Formations usually 
analyzed include sandstone and limestone. Chalk, serpentine, and con- 
glomerate analyses require specialized techniques. The depth and pressure 
of the reservoir are reflected in the residual saturation determinations. 
Cores may be of the conventional and wire-line, full-diameter, core-barrel 
type, or the smaller sidewall samples. 

Core analyses may be defined as the determination and evaluation of 
the productive characteristics of a formation sample by the measurement 
of porosity, permeability, and residual fluid saturations. Other tests in- 
clude acidization, grain size, interstitial water content, and core-water 
salinity. 

Core Sampling 

Any core-analysis report is only as reliable as the original sampling 
and treatment of the core. 

Care should be taken to select representative samples, preferably one 
sample from each foot of core recovery. If changes in the lithology occur, 
additional samples should be taken. The core should never be washed 
with water. Samples for analysis should be wiped clean of drilling fluid 
and immediately sealed from the atmosphere to prevent fluid losses. 



CORE ENGINEERS 

Reservoir Core Analysts 

SAN ANTONIO. TEXAS 



Company 






wn 1 








CORE ANALYSIS DATA AND INTERPRETATION 

(Set leelnoUt Jot rncaning of tymboh or abb-ttiaJiom) 




DEPTH 


------ 


?r«::7 


RESIDUAL FLUIDS 


^^fTJ!" 






NUMBER 


OIL 1 WATER 


REMARKS 




•/. VOL. 1 •/. PO«e 1 % rata. 





6520.5 

21.5 
22.5 
23.5 
2U.5 
25.5 
26.5 

27.5 

28.5 

29.5 
30.5 
31.5 
32.5 



191 188 
IW; 2^42 



278 
U78 
270 
985 
377 



18 
336 

76 
5 



830 loUo 

iceo 79 

856 96 

1060 872 

320 288 

186 2lt0 



22.5 
2U.0 
25.1 
2U.2 
25.U 
21*. 
2li.2 
2U.3 
21.3 
23 .U 
24.8 
21+.2 
26.U 



1.9 
2.2 
1.7 
1.9 
1.3 
1.5 
1.6 
1.7 
1.9 
0.5 
o.U 
0.5 
o.U 



U9.2 
1*9.7 
1+9.9 
1+8.8 
U9.3 
U7.6 
51.1+ 
U3.lt 
50.6 
67.5 
U7.i* 
UU.5 
U5.1 



5380 
U610 
3900 
U350 
Uooo 
1+150 
5000 
U880 

1,880 
9750 
5070 
6350 
6230 



Gas 
Gas 
Gas 
Gas 
Gas 
Gas 
Gas 
Gas 
Gas 
Gas 
Gas 
Gas 
Gas 



High water saturation 



r CORE ENSINE2R8. 



Figure 123. Tabulated core data and interpretation for well A. Although potential 
had not been run, it was probable that some oil would be produced as a spray 
with gas. 



Subsurface Laboratory Methods 



297 



The practice of breaking up the core into small pieces to smell and 
taste for the presence of hydrocarbons should be avoided. The use of 
an ultraviolet light will detect the presence of liquid hydrocarbons, and 
a portable gas analyzer will detect even minute quantities of gas. Smell- 
ing or tasting cores for the presence of gas is fallible because sweet gases 
have no apparent odor or taste, and many gas sands have been condemned 
as water productive because of the inability physically to detect gas. 











CORE ENGINEERS 

Reservoir Core Analysts 
















SAN 


ANTONIO. TEXAS 
















GAS AND WATER PERMEABILITY RELATIONSHIPS 
















EE REPORT LETTER FOR 


.ecunioNOFR 


'""••"• 
































SPECIFIC PERMEABILITIES 










PERMEABILITY 


RATIOS 








PERMEA 


BILITY. M 


LLIDARCY8 ,. 








HAT. 


OS OF SPECIFIC PERMEABILITIES 




o-»«.. 1 ...TW.™ 1 n,«„w.T.. 




DRY OAS 


1 ORT OAS 


1 FRESH WATER 


• ALT WATER 


1 PREEM WATER 


1 5ALT WATER 




£520.5 


191 




81 




71; 




2.1* 


2.6 


0.913 


21.5 


Ihh 




ko 




18 




3.6 


8.0 


O.liSO 




22.5 


278 




101 




71* 




2.8 • 


3.8 


0.733 




23.5 


hlB 




365 




322 




1.3 


1.5 


0.382 




2li.5 


270 




IC5 




100 




2.6 


2.7 


0.970 




25.5 


985 




220 




163 




1*.5 


6.0 


0.71*2 




26.5 


377 




111* 




102 




3.3 


3.7 


0.095 




27.5 


830 




3U0 




300 




2.h 


2.8 


0.882 




28.5 


loeo 




131 




130 




7.8 


7.8 


0.992 




29.5 


856 




298 




220 




2.9 


3.9 


0.738 




30.5 


1060 




325 




296 




3.3 


3.6 


0.910 




31.5 


320 




k6 




a 




7.0 


7.8 


0.892 




32.5 


186 




3k 




11* 




5.5 


13.3 


0.1*12 




AVERAGES 


538 




169 




11*3 




3.2 


3.8 


O.8I4.6 




Total 


Capacity (Cry 


Gas) 


■ 6995 


md. 


-ft. 










Total 


Capacity (Salt 


V/a te r 


) ■ 2198 


md. 


-ft. 










Total 


Csoacity 


(Fres 


h VVate 


r)= 185U 


md. 


-ft. 


















) NO TCBT. BAHm.C OrSI 












POLICYi THK IKTUIMI 


ETATIONB OR OWN 
e- TMt CLICMT. 


ION« CXR 
NORCSPC 




:;"o^T«"» 


'^.°\'^°" 


NR ANALYZED WILL ■■ 


pitcaENTeo won the excLuatv 


; 



Figure 124. Gas- and water-permeability relationships for well A. Results of these 
tests show that sand is clean and relatively free from hydratable materials. Sand 
of this type would respond to injection of gas, salt water, or fresh water. 



298 



Subsurface Geologic Methods 



Cores may be analyzed on location or preserved for off-location 
analyses. Methods of core preservation include quick-freezing and sealing 
in airtight containers such as small-diameter plastic tubes. These tubes, 
which may be obtained to fit the core diameters very closely, reduce 
the void. If the void is sufficiently small, cores may be preserved for 
indefinite periods in airtight containers. Pressure is created in the con- 
tainer by the evaporation of a very small amount of the fluids in the 
outer portion of the core; only the center of the core is used for satura- 





CORE ENGINEERS 






Reservoir Core Analysts 






SAN ANTONIO. TEXAS 




r'.nuPAKjv 


\uc,t A 
















GAS PRODUCTIVE FORMATION CORE DATA SUMMARY 






(SEE FOOT NOTES FOR EXPLANATION OF FIGURES IN PARENTHEBKSI 




FORMATION OR ZONE NO. 


1 












DEPTH. FEET 


6520 - 6533 












PROBABLE PROOUCT,0«,„ 


Gas 












ANALYZED PRODUCTIVE. 

FEET 


13 - 13 












CORED RECOVERED. 

FEET 


15 - 13 












COREPECOVERV. , 


100.0 












AVERAGE PERMEABILITY 
MILLIOARCYS «Z> 


538 












%'ir.1^tl'^.'.v^^TfZ^' 


6995 












.V.R>,O.POHCS,T.P.„C..T 


21.2 












AVERAGE POROSITY. 


1878 












SXISSS,SSn°S?iE°;^ACE 


1.3 












AVER RESID CONDENSATE 
SATURATION. % PORE SPACE 














SXI3S2f,SSTpS'R*Jrp.c.,a, 


Ii9.6 














3U 












gX'Ptv"''-^??^*''^"'' 


39 














GAS RESERVES AND RECOVERABLE GAS 






MCF PER ACRE FOOT OF FORMATION 




R.S.RVO,RR.S.RV.„. 


6.96 












SUR«CE RESERVE.,, 


1122 












RECOVERABLE GAS 161 


626 












.O.E, 






(•i HEFEB TO ntPOKT Lrrrew. 


<4| CONNATE PLUS ORILLINO WATER. ITI AT 1. T PSI ANO «>V. 




Il> rncoiCTION ASSUHU COMPLETE I 


ONE ISOLATION. 191 NON.PROOUCISLE CAPILUIRT WATER ISl AT SURFACE CONOITIONS ESTIN 




.» BP.CIF,C -E.«E».,L,rr 


...AT RESERVOIR PRESSURE .NO TE-PtR.TURE ... INSUFFICIENT RESU^OIR OAT. 


OR CALCULATIONS. 


POLICY: THE INTERPRETATIONS OR 


PINIONS KXPRKSSCO IN VMIS BSFOSff UmsSUrT THC BCST JUDOHCNT OF COKS ENOINECRS ANO ARE PRSSE 




AND CONriOENTlAL USE OF THE CLIEN 


T. NO RESPONStaiLITV AS TO TMC FWOOUCTIViTT OR PROFITASLSNCSS OP ANY ZONE ANALVZSD WILL St ASSl 


HKD ST CORK ENOINUIM. 



Figure 125. Summary of core data, gas reserve, and recoverable gas for well A. 
Capacity is sufficiently high for commercial gas production. Recoverable gas 
volume has been calculated to a residual pressure of 200 p.s.i. 



Subsurface Laboratory Methods 299 

tion tests. Once this pressure is created, further evaporation ceases. The 
practice of quick-freezing cores apparently has proved feasible. However, 
the samples selected for saturation measurements should be thawed before 
analysis because frosting occurs upon their exposure to air. The frosting 
action picks up water from the atmosphere and alters the core saturation. 
To avoid contamination from atmospheric water, frozen cores may be 
thawed in airtight containers. It thus seems logical to place the cores in 
airtight containers immediately upon sampling at the well site in order to 
avoid undue water contamination from the atmosphere while thawing and 
sampling for analysis. 

Types of Core Samples 

Normally the conventional and wire-line cores of diameter over 1^ 
inches are the best type for reliable analysis. 

Cores obtained by a shoveling or scraping action or by percussion 
bullets often prove of little value because the extent of compaction, frac- 
turing, and abnormal contamination associated with taking samples of 
these types cannot be adequately evaluated. Microscopic examinations of 
many side-wall cores has shown varying degrees of mud contamination. 
The basic fundamentals of core-data interpretation often prove valueless 
for sidewall cores of small diameter. In general, residual-water satura- 
tions of side-wall samples are higher than those for wire-line cores from 
the same sand. Special techniques have been developed in order to mini- 
mize the source of error due to the small size of the samples, but frac- 
turing and compaction alter the permeabilities of small side-wall cores. 

Physical Characteristics of Core Samples 

Porosity 

Porosity is the available void or storage capacity of the reservoir 
and may be expressed as a percentage or in barrels per acre foot. 

The effective porosity is of primary interest in the calculation of 
reserves because it is the ratio of the interconnected pore spaces to the 
total bulk volume. Porosity is a direct function of the grain size. 

There has been confusion in the industry between porosity and perme- 
ability. A formation may have a high porosity but low permeability. 

Permeahility 

Whether the fluids will flow from the formation or remain locked in 
the pores is dependent on the permeability. Conventional core-analysis 
reports usually present the "specific" dry-gas permeability of cores, for 
which the unit of measurement is the millidarcy. 

Experience has shown that the dry-gas permeability can be a very 
poor index to the productivity of so-called bentonitic sands because of 
the swelling action of fresh water on these sands.^^ Water-permeability 

" Caran, J. C, and Ca.an, R., Gas and Water Permeability Relationships of the Navarro Sand in the 
South Texas Area: Mines Mag., vol. 38, no. 12, Dec. 1948. 



300 



Subsurface Geologic Methods 



data should be a part of every core-analysis report made on sand samples. 
Clean, porous limestones usually show the same permeability to water 
as to dry gas because the measuring fluid does not react with the sample. 
Permeability may be measured parallel or vertical to the bedding 
planes of the formation. Vertical permeability should be measured for 
all formations having fluid contacts because of the importance of selecting 



CORE- LOG 

(Note: Stf daid pdgt faolmotei for explanation of ahbftvialions or ijti 

PERMEABILITY -MiLLiDARCYS O— O PR( 

1250 1000 750 500 250 IwATER-f 


bolt in parentbtses.) 

)DUCTION INDEX 0— O 




POROSITY- PERCENT • • 

, 50 40 30 20 10 q ^j.^^ 


RE< 

2C 


31DUAL OIL-% PORE SPACE • • 

) 40 60 80 100 


- _JiEiIIJJLLJC' ^t __ 






"IT ■■ ' 


(""-Ci ii-^r hi P ; -n €ab .:.: -tj 






«" * •<' Jal; Vsts- I'ernsiliilit' 


"^^f-" 












71571: 






> I ^ ' 






^-'^ 


X P]i 


I ■'*■'■ 


.ccN)i:isA.r: '■i 


i l^l. 




\\^ 


* 4V 


\ 31' 


■ ciL <:cNn_r;_s_[;i.'5.',.'?>_ 


i ^!°,-.758C 


\ 


,j^f 


i L--^tS 


■■'..'■■■■'- 


I 


vr'' 


\ % 


\ 


\ 


i A^ 


f 


iJIJii'"^ iiL \ 


— -:--"--------3--;-?'s^^-- 


k 


^^^ 


_I 2i.i_.7Cflc 




i 


\l 




^' :':■;;:• 


t 


it 


1 ; 


[ 


^ - .1- 


^ ?5 


^ 


>' .. ._ 


* r 


■M 


. n 


'.ii-)i ..-<'' ccNr.i;: 


* -'!aS_.7^Qf 


f 


cr-'" 


: : t i x<'' ^^^' 


W''--: 


i 


,.^) 


* ^c^ 


^''' 


VAT J J 


*__<?::_.§,_. 




^» 




• ^•., ^ 




3 




' ^?L.7Sqc 




■ ii 


































Veil :i 











Figure 126. Core-log of well B. 

the completion zone least likely to be affected by fluid coning. (See figs. 
126 and 127) . The degree of coning of water or gas is a function of the 
pressure distribution and the horizontal and vertical permeability ratios 
and may be calculated for any point in a thick formation if complete 
permeability data are available. ^^ Core-analysis data may be used to cal- 



' Caran, J. C, and Caran, R,, op. cit. 



Subsurface Laboratory Methods 



301 



culate the volume of clean oil that will be produced before bottom water 
cones into the sand face.^^ 

Research has determined that the presence of more than one fluid in 
the formation affects the flow of the other fluids.^*^ The apparent perme- 
ability to one particular phase of saturation in a mixture of fluids is 
called the "effective permeability." Permeability tests using highly saline, 













CORE 


ENGINEERS 
















Reservoir Core Analysts 


















SAN 


ANTONIO. TEXAS 








Co 














WELI 


B 
































CORE ANALYSIS DATA AND INTERPRETATION 












(Sif joolnoltJ for 


""■"'«" 


jymbali or ^hbr 


„a,o„) 






SAMPLE 


DEPTH 


PERMEA 


muTV 


pORosn^T 


BES 


IDUAL FL 


UIDS 


SALINITY 


productI'In 




REMARKS 





L 


1 WATER 


NUMBER 


f«T 










% PORE 


1 r. PORE 


CHLORIDES 


III 










lor. 


Vert. 
















1 


7575.5 


11.3 


67 


23.7 


0.^ 


1.3 


31.9 


39UO 


Condensate 




2 


76.5 


1'49 


107 


23.9 


0.3 


1.2 


39.7 


5000 


Condensate 




3 


77.5 


936 


298 


2U.h 


0.8 


3.2 


3U.8 


li930 


Condensate 




h 


78.5 


72 


26 


23.6 


0.3 


i.U 


21.1 


9320 


Condensate 






7579. C 


GAS- OIL no 


NTACT 














3 


7579.5 


206 


236 


2li.l 


1.7 


7.2 


30.1 


6780 


Oil 26° API 




6 


80.5 


12); 


125 


2U.8 


1.8 


7.3 


36.5 


U3U0 


Oil 




7 


81.5 


19 


% 


23.1 


2.1 


9.2 


35.3 


U070 


Oil 




8 


82.5 


151 


205 


22.5 


2.3 


10.1 


32.2 


U70 


Oil 




9 


83.5 


358 


127 


21+.5 


2.U 


9.7 


32.7 


6300 


Oil 




10 


8!i.5 


103 


0.0 


23.8 


2.2 


9.h 


27.6 


7280 


Oil 




11 


85.5 


51 


118 


22.6 


2.3 


10.0 


31.0 


6250 


Oil 




12 


86.5 


37 


22 


2U.2 


2.3 


9.5 


31.3 


7500 


Oil 




13 


87.5 


87 


58 


25.5 


2.U 


9.U 


30.5 


5000 


Oil 




lU 


88.5 
7589. C 


68 

WATEF 


60 

-OIL 


2ii.6 
CONTACT 


2.0 


8.1 


35.2 


1330 


Oil 




15 


7589.5 


6U 


128 


23. U 


1.6 


6.8 


U0.8 


7200 


Transitional 




16 


90.5 


296 


143 


2li.U 


1.2 


U.9 


liO.l 


3030 


Transitional 




17 


91.5 


210 


118 


2i|.8 


0.9 


3.6 


U6.6 


2830 


Water 




13 


92.5 


.'45U 


290 


2i*.9 


0.8 


3.3 


Ul.U 


2280 


Water 




19 


93.5 


370 


292 


2U.8 


0.8 


3.1 


39.0 


21ii0 


Water 




20 


9U.5 


33 


13 


23.0 


0.8 


3.U 


;42.6 


5380 


Water 




..,« 


U>»,TO«.0,.T.^ 


» ,H 


,HO... 


-L.. .»„ 


»0,LOW. 




O-ERC.VE. 


0„. .LP- 


LOW PLOW RATE, .C. 


ONDEN.AT.TVP..E.IDUAL. 


POLl 


■°'"IT,"!Z"^ 


toZ"lc 


PINIOMB 


EXPREIBED IN 


rHIt REPORT 


■EPPESENT 


THE BEST JUD 


SMENT OP CC 


RE ENOIHEER. AND ARE PR 


E.ENTED FOR THE E»CLUBIVB 


A«DC 




r THE CLl. 


NT. NO Rl 


.PON.l.lLlTT 




oucTivm 


OR PROP.TA.L 


NE..OF»«Y 


20NE ANALYZED WILL BE A 


SUMEO BY CORE CnoinEERB 



Figure 127. Tabulated core data and interpretation for well B. The presence of a 
gas cap and bottom water presented a problem in completion, particularly as 
vertical permeability exists at both of the fluid contacts. This well was squeezed 
three times in an attempt to shut off the gas; however, it is still producing with 
a high gas-oil ratio. The residual-oil saturation is somewhat low for normal-ratio 
production. 

synthetic brines apparently give results which reflect the effective perme- 
ability of the formation.®^ 

The ratio of the effective permeability to the specific permeability is 
termed the "relative permeability" and is usually expressed as a percentage 

^^Muskat, M., and Wykoff, R. D., Am. Inst. Min. Met. Eng. Trans., 1935. 

"' Muskat, M., Performance of Bottom-Water Drive Reservoirs: Am. Inst. Min. Met. Eng. Tech. Pub. 
2060. Sept. 1946. 

*' Bolset, Hw G., Flow of Gas-Liquid Mixtures Through Consolidated Sand: Am. Inst. Min. Met. 
Eng. Trans., 1940 



302 Subsurface Geologic Methods 

or a fraction. The relative-permeability characteristic may be used with 
other data from core analysis and reservoir-fluid analysis to predict the 
performance of reservoirs under various productive mechanisms.^- The 
relative-permeability characteristic is reflected in produced gas-oil ratios 
as the gas saturation increases, in the drop of productivity of gas-conden- 
sate wells when the sand face becomes saturated with condensate, in the 
drop in potential of gas wells when water is produced through the com- 
pletion interval, and in the production of water from zones of high connate- 
water saturation. 

A lower limit for commercial permeability cannot be set because the 
factors of sand thickness and reservoir pressure directly affect this limit. 
The extremely low permeabilities of some formations in Colorado, Wyo- 
ming, and Texas are compensated for by the great thicknesses and high 
pressures. Actually it is not the permeability of any one foot of formation 
that is so critical; it is the total capacity (the permeability times the thick- 
ness) together with the reservoir pressure that controls the flow rate. 

Residual-Fluid Saturations 

The residual-liquid saturations determined by core analyses are the 
oil or condensate and the total core water. (See figs. 123, 127, and 131.) 
The gas volume is determined indirectly as the diff"erence between the total 
pore volume and the liquid volume. These saturations may be determined 
by extraction or retort methods. One of the most rapid and efficient 
methods is the electric, water-cooled condenser-fluid stills. 

Residual Oil and Condensate 

The amount of residual oil remaining in the core sample cut from 
a high-pressure reservoir is dependent on the following factors.^^ 

1. Reservoir-liquid saturation. 

2. Formation or reservoir pressure. 

3. Mud pressure and water loss. 

4. Reservoir-liquid viscosities. 

5. Coring time. 

6. Vertical and horizontal permeabilities. 

7. Pressure-depletion rate while pulling core. 

8. Core diameter and type of core. 

9. Method of obtaining core sample. 
10. Solution gas-oil ratio. 

The residual hydrocarbon saturation is dependent on the flushing or 
contamination by the drilling fluid that takes place as the formation is 
cored. When coring with water-base mud, flushing takes place ahead of 
the bit and radially through the core, owing to the difference between 



^- Muskat, M., and Taylor, M. O., Effect of Reservoir Fluid and Rock Characteristics on Production 
Histories of Gas-Drive Reservoirs: Am. Inst. Min. Met. Eng. Tech. Pub. 1914, 1945 

'^ Caran, J. G., Core Analysis — Its Interpretation and Application in Reservoir Engineering : Petroleum 
Eng., Oct. 1947. 



Subsurface Laboratory Methods 



303 



the mud weight and the formation pressure. While the core is being 
brought to the surface, gas expansion takes place with the reduction in 
pressure, thus driving out additional oil and gas; by the time the perme- 
able cores reach the surface the pressure depletion is complete. 

The residual-condensate saturation is the result of the retrograde- 
condensation mechanism that takes place as the pressure is reduced. 

The residual oil or condensate may be expressed in percentage by 









CORE ENGINEERS 














Reservoir Core Analysis 
















SAN ANTONIO. TEXAS 


















GAS AND WATER PERMEABILITY 


RELATIONSHIPS 










nOMPiNV 








WE 


t, B 






























SPECIFIC PERMEABIUTIES 






PERMEABILITY 


RATIOS 






1 


0,Y »>. 


PER»€AOILlTY M 


IXIOARCYS Ml 




tR* 


TIOS OF SPECIFIC RE 








1 ,.LTW., 


x. 1 ,i...-w.r„ 1 


DRY QAS 


1 ORY OAE 


1 FRESH WATER 




5ALT WATER 




1 SALT WATER 




7575.5 


lli5 


U2 


35 


3.U 


U.l 


0.83U 






76.5 


Ui9 


5i4 


1^6 


2.8 


3.2 


0.852 




77.5 


236 


113 


9U 


2.1 


2.5 


0.831 


. 


78.5 


72 


29 


16 


2.5 


U.5 


0.552 




79.5 


206 


127 


111 


1.6 


1.9 


0.875 




80.5 


12U 


26 


16 


U.B 


7.8 


0.615 




81.5 


U9 


10 


2 


h.9 


2I..5 


0.200 




82.5 


151 


51; 


12 


2.8 


3.6 


0.778 




83.5 


358 


212 


185 


1.7 


1.9 


0.872 




8U.5 


l<3 


39 


17 


2.6 


6.1 


O.U36 




85.5 


51 


Ih 


9 


3.6 


5.7 


o.6h5 




86.5 


57 


7 


5 


5.3 


l.h 


0.7 Hi 




87.5 


87 


33 


20 


2.6 


u.u 


0.606 




88.5 


68 


11 


9 


6.2 


7.6 


0.818 




89.5 


^4 


12 


10 


5.3 


6.U 


0.833 




90.5 


296 


220 


209 


1.3 


l.U 


0.950 




91.5 


210 


189 


175 


1.1 


1.2 


0.925 




92.5 


h5U 


178 


99 


2.6 


U.6 


0.556 




93.5 


370 


129 


63 


2.9 


5.9 


0,U88 




9U.5 


33 


3 


2 


10.0 


16.5, 


0.667 




AVERAGES 


163 


75 


58 


2.2 


2.8 


0.773 






TELTSAT^, 




,~OTE.T,»-.n.CO„.N,ECI,.TEO , 










POLICY: THE INTWIFItCTATr 


SN.OP.OP. 


N.ON. Exr.».co m T 


Xia «Ero«T .ir.ESENT THE BEET JOO 


»ENT OF CO 


RE EN.mEER. AHO ARE 


FREJEHTEO FOR THE EXCL 


URIVE 


, »HO CONF,D.HT..L U.. or 


». CUtNT 


"'■""""'•'•"•'"» 


. TO T». ►.OOUCn.lTT OK I.ROF.T..LE 


.NEMOFAKY 


.ONE A»ALY,EO WILL .E 


AERUUEO BY CORE Enoin 


EER. 



Figure 128. Gas- and water-permeability relationships for well B. The fresh-water- 
salt-water permeability ratios are somewhat irregular. It is probable that this 
sand would respond most readily to gas and salt-water injection. 



304 Subsurface Geologic Methods 

volume, percentage of the pore space, and barrels per acre-foot of for- 
mation (figs. 127 and 131). 

Residual Core Water 

In high-pressure, flush reservoirs cored with water-base mud, the 
amount of residual or total water in the core at the time of analysis is 
the sum of the connate plus any drilling water that may have been forced 
into the pores of the sand while the core was being cut. 

If the formation contains minerals characterized by a high chemically 
bound water content, this water will also be recovered when high-temper- 
ature-retort methods are used, and corrections for such water of crystal- 
lization should be made. Low-temperature extraction methods seldom 
recover water of crystallization. 

Cores cut with oil-base mud from low-pressure reservoirs normally 
show total water saturations that may be assumed to be the connate or 
interstitial water saturation of the formation. Deep, high-pressure reser- 
voirs cored with oil-base muds show total water saturations that may not 
be the true interstitial-water content because of the high temperatures en- 
countered. 

The total core water is usually presented in percentage of the pore 
space. 

Connate Water 

Connate water is present in varying degree in all water-wet sand or 
lime formations. It has been termed both "interstitial" and "connate" 
water. In the writer's opinion, the term "connate" water is the more 
desirable and will be used throughout this discussion. 

Connate water is defined as the water in the formation at the time 
the formation is cored. It is immaterial in core-analysis work whether 
it is the same water that saturated the sand when it was first deposited 
or water that migrated into the sand during later geologic time. The 
connate water may be made up of "free" water, and the water may be held 
in the interstices of the sand by capillarity. The free water may be pro- 
duced, but the capillary water is not producible. Most oil or gas sands 
are water-wet and contain some connate water, although water-free fluids 
may be produced. 

A number of methods are available for measuring or calculating the 
connate-water saturations of sand formations. 

1. Capillary pressure versus water saturation by gas-pressure and 
centrifugal methods. ^^ 

2. The use of tracers in the drilling fluid.^^ 

3. The calculation of saturation based on electric-log resistivities.^® 



=>< McCullough J. J., Albough, F. W., and Jones, P. H., Determination of the Interstitial-Water Con- 
tent of Oil and Gas Sands by Laboratory Tests of Core Samples: Am. Petroleum Inst. Drilling and 
Production Practice, 1936. / o r c j » 

i^ Pyle, H. C, and Jones, P. H., Quantitative Determination of Connate-Water of Oil Sands: Am. 
Petroleum Inst. Drilling and Production Practice, 1936. 

58 Jones, J. P., Water Saturation vs. Resistivity: Petroleum Production, vol. 1, pp. 54-56, 1916. 



Subsurface Laboratory Methods 



305 



4. Coring with oil-base mud.^'^ 

5. Calculations based on salinities of core water.^^ 

6. Empirical calculations based on the porosity and residual fluid 
saturations.^^ 





CORE ENGINEERS 






Resei ion Core Aiialyjis 






SAN ANTONrO. TEXAS 




r-r,UDAKjv 




WELI B 








CORE DATA SUMMARY 


ZONE OR DIVISION NO 


V/ilcox 














DEPTH. FEET 


7579 - 7^89 














PROBABLE PRODUCTION (I) 


Oil 














ANALYZED PRODUCTIVE. 


10 - 10 














CORED. FEET 


10 














CORE RECOVERY % 


100.0 














AVERAGE PERMEABILITY. 


123 














SV.TVo^JJt^V^'i'.^T"'^^' 


123U 














AVERAGE POROSITY PERCENT 


2U.0 














AVERAGE POROSITY. 
BARRELS PER ACRE FOOT 


1862 














SATURATION. % PORE SPACE 


9.0 














JXISSSLSS'^^^oITe^Ipace 


32.2 














SATURATION. CALCULATED 


21 














RESIDUAL OIL GRAVITY. 'API 


26 














FORMATION VOLUME FACTOR. 
ESTIMATED 


1.51 














SOLUTION GAS. OIL RATIO 
CUBIC FEET PER BARREL 12) 


680 














OIL IN PLACE 

BARRELS PER ACRE FOOT ( 3 ) 


liiTl 














RECOVERABLE OIL. STOCK TANK BARRELS PER ACRE FOOT 


EXPANSION (4) 


Uko 














INCREASE BY EFFECTIVE 


373 














MAXIMUM RECOVERY. 
AFTER WATER DRIVE (5> 


813 














trort 






(•) REFER TO REPORT LETT 
12) PRESSURE, REDUCTION 


ER. 14) AFTER PRESSURE R 


EDUCTION FROM 
RE TO ZERO PSI. 

R PRESSURE MAINTAINED 


<3) OIL VOLUME AT ORIGIN 


AL RESERVOIR CONDITIONS. (6) CAS PHASE RESERV 


OIR: NO ESTIMATE. 


POLICY: THI IKTE«PPirrATIOK« Oil C 


PINION! CxraUlBO IN THIS ttXPOKT RBPIttSCHT THI SIST JUOSUINT OP COfU ENaiNKXKS AND A 


I PHUINTD FOR THI IICLUaiVI 


AW> cohubihtial u« or th. cl..« 


T. NO ■UPONCIBIUTT At TO THt PHODUCTIVITY O* PROPITABLKNCM OF AMY ZOMI ANALYZSO WILL 


■■ Auuyio .Y CO.. ENOiNim. 



Figure 129. Summary of core data, oil reserve, and recoverable oil of well B. Be- 
cause of the gas cap and the probability of a water drive, it is apparent that 
the ultimate recovery will be higher than the solution gas recovery indicated. 



■" Stuart, R. W., Use of Oil Base Mud: Oil Weekly, pp. 41-45, May 27, 1946. 

^' Sage, H. F., and Ai.ustrong, D. M., Estimation of Connate Water from the Salt Content of the 
Core: Am. Petroleum Inst. Prod. Bull. 223, May 1939. 

** Lewis, J. A., Core Analysis — An Aid to Increasing the Recovery of Oil: Am. Inst. Min. Met. Eng. 
Trans., pp. 68-75, 1942. 



306 Subsurface Geologic Methods 

Experience has shown that the total water saturations measured for 
limestone cores from oil fields in Caldwell, Guadalupe, and Milam Coun- 
ties in south Texas may be considered as the connate-water saturations. 
Cores cut with diamond-core bits using water-base mud normally show 
total water saturations between 15 and 25 percent of the pore space when 
the cores are oil-productive. 

The practice of using dextrose in cable-tool coring of partially de- 
pleted oil sands in some Pennsylvania fields has proved practical in differ- 
entiating between drilling, connate, and flood or extraneous water.^"^ 

A recent development of the capillary-pressure studies shows possi- 
bilities of using the high-pressure mercury pump in measuring that por- 
tion of sand-core samples occupied by capillary water.^ 

The use of capillary-pressure measurements for the determination of 
cgpillary water is commercially practical. Of particular value are the 
restored-state techniques in that the core samples need not be fresh. It is 
highly desirable to know the height above the water table when selecting 
the portion of the pressure curve applicable to the particular zone being 
tested. The portion of the curve which represents the irreducible-minimum 
water saturation may not apply to all reservoirs and is dependent on the 
closure. In practice, a core more than fifty feet above the water table in 
the reservoir will probably contain its minimum water saturation.^ 

A series of comparison tests in which the connate-water content of 
a large number of cores cut with oil-base mud was compared with results 
obtained by the following methods showed reasonably close agreement; 
these included the use of the capillary-pressure method, calculations from 
electric-log resistivities, calculations based on salinities, and distillation 
measurements.^ 

The salinity of the residual water in cores cut with water-base mud 
has been used in an attempt to estimate the connate-water saturation. 
This method has been found to be unreliable for most cores taken from 
flush, high-pressure reservoirs because of the contamination by the water 
in the drilling fluid. Zones of very low permeability may show salinities 
within the range of the formation water, but connate-water content calcu- 
lated from these salinities may not apply to more permeable flushed zones 
because of the relationship between permeability and connate water. Com- 
parison tests using electric-log, oil-base, and tracer data have established 
a logarithmic relationship between permeability, total water, and connate 
water.^ Unfortunately, no one relationship will apply to all sand bodies. 



*'"' Clark, A. P., A Method for Determining Connate and Drilling Water Saturation for Cable Tool 
Cores: Producers Monthly, July 1947. 

' Purcell, W. R., Capillary Pressures — Their Measurement Using Mercury and the Calculation of 
Permeability Therefrom: Am. Irst. Min. Met. Eng., Fall Meeting, Dallas, Texas, Oct. 1948. 

^ Bruce, W. A., and Welge, R. J., The Restored State Method for Determination of Oil in Place and 
Connate Water: World Oil, Aug. 1947. 

' Thornton, 0. F^, and Marshall, D. L., Estimating Interstitial Water by the Capillary Pressure 
Method: Am. Inst. Min. Met. Eng. Tech. Pub. 2126, Jan. 1947. 

* Earlougher, R. C, Core-Analysis Problems in the Mid-Continent Area: Am. Petroleum Inst. Drill- 
ing and Production Practice, 1940. 



Subsurface Laboratory Methods 



307 



CORE 

(Note: Sit d4l4 ptgt Icelnult: Icr txpUnalia 

POLICY- TH. 1MT.I1PI..T.T.0N» on O'.N. 

PERMEABILITY -MILLIDARCYS O— O 


-LOG 

n o/ abhrtv'miotit or symbols it pdrenthtsft,) 

PRODUCTION INDEX 


t>— o 

,1 


1 1 1 1 1 1 1 II 


1 ""^ " ""•' 


J 


POROSITY- PERCENT •— -• RESIOU 

5,0 40 3p 20 10 q 20 


AL OIL-% PORE SPACE 
40 60 60 I0( 


•- — • 
J 




















^^ __t 














';jt:EV<:iirf JIT _ ,q,q . 






<>--<» Oil/ }(,; P; -r;( ati L'.' ij \ / 






<> ""O ! alt Ut -ter ^'f r -ne i ):,! it r 


\ / _ 








A 








A 






T m "V J , . 














■—■■^ 

















___ 






)* 1^'']l, loeo'" -^^---1^ 


(} 




1— ij-t---;:^;:/ J i<^V..- t^ 


1> 




c« ■ "■ ■ "'^" c' 


-'■'■ "^5 


'"■■■■ 


? 


"l::=»= ^ 


*-'^ \. 


( 




__^.,. -^ 


■ :: I 


\t < 


L 


7 .-.^•"""'^ > T roc- ■ - _L 


f 


>' 


-; = r|=. = - f-'^l.-. „i 


i1 




^L 'L .■^' 


^-- I 


li 




^ -J 


■-;.: 


.* (IL I 


^•.- 


"""";:-h'" \ 


--"•- „ '^!- 




y 


'^«=:' ^ 1 j_i IK 


■- •'■"■■> !_ 


SC° L]'] T 


< 


3») (I I ' ■: 1 M 1 M 1 1 W 




} 


--l73i '«, £_ 


1 


--- 


-■<^ 


--2^n % c:^ 


•.::■:'- 


> '■'■ail 




■•■•J 8 ''i 


■■--i _ r 


"'■• 


^i,._ 






> 


.^.„„,.._._^__ _.l(95-_,.- ... 


;::::: <: 




_, _,, _^ 


— : '*^ 


}?___._ 1= 


::»., 





'-■'■■- *'■'.' 


.i«:: 




i_::::::3-:::"c'' 


~1 


^ 


-■•<) 


;)--" j cj"' irtiO'^ -J- 


» 


_.iL 


i i\ '3 ^^ rV---: I 


T"""""t" 


> 






i 


,/ 


c- ' ^ 5 


t 


i 


,?' 


"*te-.. « -- i 


:•-■■ l\ 


( 


/ 


' -_j 1 1 1 1 1 1 1 1 1 








1 








~ 
















hM 


; 













Figure 130. Core-log of well C. 



308 



Subsurface Geologic Methods 



In general, the higher the permeability, the lower the connate-water satur- 
ation. 

A large part of this section has been devoted to the determination and 
discussion of connate-water saturation because it is considered the critical 
key in data interpretation and the calculation of hydrocarbon reserves. 













CORE 


ENGINEERS 
















Reservoir Core 


Analysts 


















SAN 


ANTONIO 


. TEXAS 








Co 














WELL_ 


c 
























CORE ANALYSIS DATA AND INTERPRETATION 


r.lY. 


°r," 


":- 


r^RcVr 


;°:?.:; 


RESIDUAL FLUIDS 


CMLoa'roes 


pZITct^on 


.EM..KS 


01 




Tp^« 


% VOL 1 


V. PORE 






tior. 


Vert. 
















1 


1C19.5 


100 


1U8 


33.9 


3.6 


10.6 


59.6 


2,060 


Oil 


Shaly, ira 


2 


20.5 


237 


200 


37.1 


/1.2 


11.6 


61.2 


3150 


Oil 


Shaly, LFR 


3 


21.5 


1260 


820 


321.7 


8.6 


26.3 


32.7 


242+0 


Oil 




h 


22.5 


570 


855 


33.9 


10.6 


31.1 


31.1 


2,800 


Oil 




5 


23.5 


990 


1370 


35.9 


13.3 


36.9 


31.9 


2,200 


Oil 




6 


2!t.5 


273 


91 


31^.9 


12.2 


321.9 


27.9 


24,90 


Oil 


Shaly 


7 


25.5 


1060 


1070 


35.2+ 


11.5 


32.6 


33.3 


3,810 


Oil 




8 


26.5 


1100 


910 


36.0 


12.0 


33.2i 


28.2 


2.530 


Oil 




9 


27.5 


1090 


]ii00 


3U.8 


13.1 


37.5 


30.2 


3800 


Oil 


20 deg. API 


10 


28.5 


1085 


L'llO 


38.2 


9.2 


22i.O 


28.7 


3610 


Oil 




11 


29.5 


iWo 


1360 


38.3 


11.8 


30.8 


30.5 


2,52,0 


Oil 




12 


30.5 


1085 


1020 


39.9 


11.2| 


28.5 


25.9 


2,330 


Oil 




13 


31.5 


1585 


1200 


36.2i 


12.1 


33.2 


28.7 


3970 


Oil 


Shale inclusions 


1h 


32.5 


2320 


1670 


33.7 


13.6 


'iS^^ 


2*0.9 


36UO 


Oil 


High wtr. sat., shly. 


15 


33.5 


1290 


1295 


36.9 


9.7 


Ms 


32.6 


2,000 


Oil 




16 


3U.5 


1220 


122+0 


38.0 


8.5 


22.1; 


30.7 


2,200 


Oil 




17 


35.5 


325 


35U 


31.5 


8.2; 


26.8 


2t3.6 


5.000 


Oil 


Shaly 


18 


36.5 


568 


U12 


32^.3 


I2i.6 


242.6 


22;.5 


5.1,80 


Oil 


Shaly 


19 


37.5 


132 


62 


32.7 


3.2i 


10.2+ 


71.8 


6.260 


Oil 


Shaly, LFR 


20 


38.5 


U80 


356 


3)i.8 


13.9 


33.9 


26.9 


6360 


Oil 




21 


39.5 


12)t0 


I;22 


32i.5 


11.9 


3h.3 


23.6 


7.82,0 


Oil 




22 


Uo.5 


1270 


950 


35.8 


13.9 


38.8 


22,. 1 


6130 


Oil 




83 


i;1.5 


U85 


6U0 


36.8 


13.2 


35.8 


35.8 


2,730 


Oil 




?J* 


I42.5 


i;i2o 


700 


36.9 


12.8 


3Ji.6 


38.5 


5960 


Oil 




?5 


U3.5 


1170 


U08 


38.5 


9.9 


25.7 


38.5 


6120 


Oil 




.', m 


r„ TO ...,■. .t^ 


, ,» 


S, NO,.« 


P.. .Hn ^ 


orLOW . 




-„C,..rL 


,„ ,LP» 


LO«.LOW,.,. 


-"— 


POLIC 


I™^!J 


T::z 


rrl 


Ii,v,°,r,: 


."'"IT^o" 


I'c".;;: 


',"";."" 


::::::." 


vj:z""zi:: 


;r;/;;:r.rcV:A".== 



Figure 131. Tabulated core data and interpretation for well C. The permeability 
to dry gas is relatively high. Some samples showed a higher permeability 
vertically than horizontally. In general the permeability distribution is good. 
The residual-oil saturations are normal for water-free-oil production from this 
sand. 



Core-Data Interpretation in Regard to Production 

The basic core data are of little value unless correctly interpreted 
by qualified core analysts. By designating fluid contacts it is possible to 
minimize completions in water sands. For example, if a six-inch perme- 
able-water-sand zone is included in the completion interval, fluid from this 
sand can drown as much as ten feet of gas or oil sand. Because of the 
critical nature of completing oil wells with a gas cap or bottom water, it 
is highly important that core depth and perforation or completion depths 
agree. Many times the core analyst is held responsible for poor comple- 



Subsurface Laboratory Methods 309 

tions when the fault lies in the disagreement between driller's and electric- 
log depths. Cores are usually submitted to the core analyst with the 
driller's depths designated. Electric-log depths may differ by several feet 
from the driller's depths, and, unless the correction is specifically made, 
the core-analysis report will be in error as to depths. The writer strongly 
emphasizes the importance of correct depth designation. 

Careful note should be made as to the type of drilling fluid used. 
Special muds such as low-water-loss, modified-starch drilling fluids are 
often used in coring sands with low productive capacity. Total water 
saturations in the same sand may vary with the type of water-base drilling 
fluid used, and this factor nmst be taken into consideration in the data 
interpretation. It is readily apparent that the practice of washing a well 
with water after drilling or coring with low-water-loss mud defeats the 
purpose for which these special muds were used. Wells of this type should 
be washed and cleaned with crude oil. In other words, the completion 
techniques employed by the operator may nullify accurate work on the 
part of the core analyst. 

Failure properly to cement or squeeze gas zones above an oil sand 
will result in high gas-oil-ratio production (fig. 127) . Sand bodies drilled 
into bottom water must be squeezed or water will be produced (fig. 127) . 
The use of excessively high pressures to break down the sand while 
cementing may result in sand fractures which will cause channeling and 
bad completions. The necessity of squeezing the same sand several times 
in order to shut off gas or water may permanently damage the sand and 
result in abandonment of the well. 

No definite set of rules or tables can be made for the interpretation 
of core data. As a matter of fact, the practice of some analysts in making 
interpretations as to probable production from the data without seeing 
the cores often leads to bad predictions. 

The interpretation of data from flush, deep, high-pressure reservoirs 
is entirely different from the interpretation of core data of samples cut 
from shallow, low-pressure, depleted reservoirs. However, there are some 
characteristics which enable the interpreter to predict correctly the type 
of production to be expected and to locate the fluid contacts.^ 

Gas Sands 

The following physical characteristics are associated with permeable, 
high-pressure gas sands: 

1. Low water saturation dependent on permeability, formation pres- 
sure, and degree of contamination by drilling water. 

2. Residual-oil saturation absent or present in varying degree, de- 
pending on the gravity of the oil and the height above the gas-oil contact. 
Residual-oil color will vary from amber to yellow. 

3. If thoroughly flushed, the salinity of the core water will approach 

^ Caran, J. G., Core Analysis — An Aid To Profitable Completions : Mines Mag., vol. 37, no. 2, 
Feb. 1947. 



310 Subsurface Geologic Methods 

that of the drilling fluid; if not, the salinity will be normal or comparable 
to that of cores from the oil column. 

4. Characteristic gas odor when fresh cores are broken. It is im- 
portant to note that some gases have no apparent odor particularly when 
composed primarily of methane and ethane, so that the lack of odor is 
not always conclusive. 

5. An inspection of the mud sheath on the core may show evidence 
of gas breaking from the pores of the sand. This is particularly true of 
sands with low permeability. 

6. Connate-water saturations are usually less than forty percent of 
the pore space. 

It is particularly difficult to differentiate between water- and gas- 
productive sands when the reservoir pressure is very low. Low-pressure 
gas sands may show total water saturations approaching 80 percent of 
the pore space and still produce dry gas. 

Condensate Sands 

Gas-condensate or distillate sands from high-pressure reservoirs may 
be recognized by characteristics similar to those for dry-gas sands, but 
with the following differences: 

1. The high gravity of residual liquid hydrocarbons, usually above 
50° A.P.I. 

2. Characteristic water- white color of the residual hydrocarbons; 
saturation will vary between two and five percent of the pore space. 

Experience is the critical factor in detecting gas-condensate sands, 
because the fluids retorted from sands with a high-shale content will 
sometimes show a light-colored meniscus at the top of the water, which 
may be taken as a condensate residual. 

Oil Sands 

Oil sands are not usually flushed by drilling fluids to so great an 
extent as gas or water sands of comparable permeability, owing to the 
influence of interfacial forces and the difference in the viscosities of the 
fluids. Some low-pressure sands with very little solution gas and satu- 
rated with oil of very low gravity may show very little flushing by drilling 
fluid. 

The residual-oil saturation of an oil-productive sand will vary with 
the factors discussed earlier in this paper under the heading "Residual 
Oil and Condensate." 

Table 17 presents a relationship between the residual-oil gravity and 
the normal saturation for oil sands having a reservoir pressure between 
2,000 and 3,500 p.s.i. An inverse relationship is evident from table 17; 
the higher the oil gravity, the lower the minimum residual oil content for 
water-free production. For shallower sands with lower formation pressure, 
the minimum saturation limits will be higher than those shown in the table. 



Subsurface Laboratory Methods 311 

It should be noted that these saturations are based on the assumption that 
no free gas will be produced from the sand face. The key to high gas-oil- 
ratio production lies in the residual-oil saturations, assuming water will 
not be produced. 

TABLE 17 
Residual Oil Saturation of Oil Sands 



Oil gravity 
(°A.P.I. at 60/60) 


Residual oil (minimum) 
(percent pore space) 


48-40 


5-10 


40-30 


10-15 


30-25 


15-20 


25 or less 


20 or more 







The connate-water saturations of oil sands vary appreciably with the 
permeability and structural position of the well. Clean sands with per- 
meabilities above 1,000 millidarcys may have connate- water saturations 
less than 15 percent of the pore-space. 

The maximum connate water that an oil-bearing zone can contain 
and not produce water varies with the effective permeability to each fluid 
phase. The limiting or critical water saturation must be determined for 
each sand in each reservoir. 

The critical oil-water ratio may be used as an index to the probable 
production. This ratio varies for different sands, and no one ratio has 
been determined that will apply to all formations. 

Bleeding Cores 

Since cores were first taken, the geologist has always been wary of 
so-called bleeding cores or cores that show oil or gas seeping from the 
pores for some time after the core is taken from the barrel. Many oil- 
bearing sands have been condemned as probably water-productive because 
of this bleeding. 

In the writer's opinion, cores bleed because the pressure-depletion 
process is not complete by the time the core is taken from the barrel. 
Usually bleeding cores are low in permeability, and thus the pressure de- 
pletion process takes longer. Formations should not be condemned be- 
cause of bleeding alone; core analyses can often explain this phenomenon. 

Oil-Saturated Limestone 

The analysis of limestone varies somewhat from that of sand or sand- 
stone in that the permeability characteristic is not so critical, particularly 
if commercial porosity exists. This discussion is limited to porous and 
does not concern fractured limestone. 

Care must be exercised in selecting representative samples of lime- 
stone for analysis because of abrupt changes in porosity. Cores from the 



312 



Subsurface Geologic Methods 



Edwards limestone in Caldwell and Guadalupe Counties in south Texas 
often show as much as 45 percent residual-oil saturation. Where oil pro- 
ductive, this limestone averages 20 percent in total-water saturation. 
As discussed elsewhere in this paper, it is assumed by core analysts in that 
area that the total water is the connate-water saturation. Porosity char- 











CORE ENGINEERS 














Resi'tioiT Core 


Aiialysli 
















SAN ANTONIO 


TEXAS 














GAS 


AND WATER 


PERMEABILITY RELATIONSHIPS 








COMPANY 














WELL 


p 






























SPECIFIC PERMEABILITIES 








PERMEABILITY 


RATIOS 




'■E!?' 




EBMEA 


BILITV M,L 


LIDARCl 


s ,„ 






(.ATI 


OS OF SPECIFIC P 


RMEABILITIES 






1 




1 








DRV CAB 


1 oRy OAS 


1 FRESH W.TER 






DRV QA9 


1 


SALT WATC 


" 1 






SALTWATEB | rBE3„ WATER | SALT WATtR 






1019.5 


100 




o-U 




0.0 




250 


(•) 


0.0 




?C.5 


237 




U.o 




0.0 




59 


(O 


0.0 


?1.5 


1260 




2(9. 




5.7 




6 


221 


0.028 


22.5 


570 




192. 




0.0 




3 


(») 


0.0 


23.5 


990 




51*. 




0.0 




16 


(•) 


0.0 


21.5 


273 




3.3 




0.3 




83 


910 


0.091 


25.5 


lo6o 




1*7. 




0.9 




23 


1178 


0.019 


26,5 


1100 




50. 




0.0 




22 


(*) 


0.0 


27.5 


1090 




108. 




i*.8 




6 


23 


0.026 


2^.5 


1085 




1*0. 




0.2 




23 


51*20 


o.oci* 


29.5 


11*70 




i*.3 




0.0 




31*2 


(*) 


0.0 


?o.5 


1085 




35. 




0.0 




31 


(*) 


0.0 


31.5 


1585 




90. 




0.0 




18 


(•) 


0.0 


32.5 


2320 




210. 




6.0 




11 


387 


0.029 


33.5 


1290 




6. 




0.1* 




161 


3220 


0.C50 


3U.5 


1220 




19. 




2.3 




61* 


530 


0.121 


35.5 


325 




7. 




0.0 




1*7 


(•) 


0.0 


36.5 


56a 




79. 




1.1 




7 


52 


O.Oll* 


37.5 


132 




7. 




0.0 




19 


(•) 


0.0 


38.5 


U80 




110. 




1.0 




1* 


1*80 


0.009 


39.5 


12i;0 




760. 




36. 




2 


35 


0.01*7 


U0.5 


1270 




61*2. 




21. 




2 


61 


0.033 


U.5 


1*85 




110. 




0.8 




I* 


606 


0.007 


!i2.5 


11*20 




121. 




10. 




12 


11*2 


0.083 


)43.5 


1170 




11*8. 




0.1 




8 


11700 


0.001 


AVERAGES 


951* 




126. 




3.6 




8 


265 


0.029 


Tot« 


1 rapacity 


(Dry 


Gas) 


- 


25325 md 


-ft. 








Tots 


1 Caoacity 


(Sal 


t Water 


) = 


311*0 md 


-ft. 








Tote 


1 Capacity 


(Fresh Wate 


r). 


91 md 


-ft. 




























POLICY: THt iNTlR.n 


.T«T,ON.O.O..N,< 


N. I>^ 


,„„ ,H TM 


• mran ...iiiiinT thi 


■ I.T JUDOM 


KT 0. CO.. 


E»a.H..>. .NO .n 


RRIIINTID FOR TH. HCLU.IVH 


.HO CO~|..O.NT,»l. u. 


°' '"■ '"■" " 


0.»rON..>,UTY.. 


TOTHI-, 


™""="""°"' 


.0|.,T..L.N. 




NB.NALYZCO WILLB 


A..uy.O .Y CORt ENQ.N.IRR 



Figure 132. Gas- and water-permeability relationships for well C. These data em- 
phasize the importance of making water-permeability tests. The sand would 
favorably respond to gas injection. Some portions of the sand would respond 
to salt-water injection, but fresh water will cause swelling of the hydratable 
materials in the sand, and restriction of flow rates will result. 



Subsurface Laboratory Methods 313 

acteristics for the Edwards limestone along the Balcones fault trend in 
south Texas vary from 15 to 35 percent. 

Where the oil exists in fractured limestone, core analyses have very 
little value. 

Chalk and Serpentine Production 

Production from fractured chalk or serpentine occurs from the frac- 
ture planes and not from the dense, impermeable portions of the forma- 
tion. Analyses made on the Austin chalk in Frio County, Texas, have 
proved of little value to the oil operator because the saturation exists in 
fracture planes. Where the saturation occurs in solution porosity in these 
formations, the same techniques used in limestone analyses apply. 

Conglomerate Production 
Analyses made by the writer on conglomerate cores from the north 
and north-central Texas areas show very high permeabilities. The porosity 
varies with the sorting and shape of the materials making up the conglom- 
erate. Where the pebbles are of uniform size and cementation is at a 
minimum, the porosity approaches a maximum. Usually, conglomerate 
cores are thoroughly flushed by drilling water, and the estimation of the 
connate-water saturation becomes a problem. Minimum-reserve calcula- 
tions based on analyses of consolidated conglomerate may be made using 
total water saturations as the connate water. 

Water Sands 

Water-productive sands associated with oil sands may be recognized 
by low and irregular oil saturations in the transitional zones (fig. 127). 
Hydrocarbons may not be present below the oil-water contact. The oil- 
water contact of a uniformly permeable sand is defined as the level in 
the sand column below which water alone will be produced. The total 
water saturations of water sands are usually higher than those for com- 
mercially productive sands and will approach 100 percent of the pore 
space when hydrocarbons are absent. Sands that produce water contain 
large amounts of "free" or noncapillary water. 

Transitional Zones 

Where a gas cap or bottom water is present in a thick sand column, 
assuming an oil column exists, there are transitional zones from the gas 
to the oil zones and from the oil to the water-productive zones. These 
zones vary in thickness within the same reservoir. 

The gas-to-oil transitional zone may be recognized by an increase 
in residual-oil content as the gas-oil contact is approached. It is advisable 
to confine the completion interval to the oil zone and several feet below 
the designated gas-oil contact in order to minimize the possibility of gas 
channeling and high gas-oil-ratio production. 

The transitional zone from oil to water is usually characterized by 



314 Subsurface Geologic Methods 

an increase in total water content and a small drop in residual-oil satu- 
ration. The oil-water ratio will be smaller than that for the oil-productive 
zone. This type of transitional zone is of doubtful commercial value; 
usually, water is produced with the oil upon initial completion, and, as 
the oil saturation is decreased, the permeability to water rapidly increases 
to the point where the entire zone is water-productive. 

The foregoing discussion emphasizes the importance of making ver- 
tical permeability tests. Vertical permeability barriers should be used in 
selecting the completion interval. Many core-analysis reports contain no 
vertical permeability data because some core analysts have not recognized 
the importance of these tests. The writer believes that every possible test 
should be made on core samples while the cores are available because, 
once the cores have been discarded, these additional data can only be 
secured by recoring the formations. 

Secondary Migration of Water 

There is no way of recognizing oil- or gas-sand zones which have 
been subjected to the secondary encroachment of water, and many wrong 
predictions as to probable production may be ascribed to this phenomenon. 
Cores from these zones can show exactly the same residual hydrocarbon 
saturations as the cores from zones that have not been flushed by salt 
water. 

The reason these zones cannot be recognized is that there is sufficient 
gas left in the sand to drive out the excess water and thus make the water 
saturations similar to those of gas-productive sands. Checks by the use 
of the gas analyzer would show gas to be present. The only conclusive 
test is to make a production test through the drill stem. Flooded oil sands 
can show normal residual-oil saturations and still produce nearly 100 per- 
cent salt water. 

The inability of the core analyst to recognize these zones solely on 
the basis of core analysis should be recognized by the oil operators, and 
all of the data as to the structural position of the well, the probability of 
faulting, and electric-log correlation should be made available to the in- 
terpreter. It is sometimes difficult to understand why the oil operators 
withhold this information from the core analyst when they are paying for 
the work. Any and all pertinent data that will help in arriving at the 
right answer should be used. 

Oil in Place 

All of the oil in place cannot be recovered by any known means of 
production. This volume is usually calculated at reservoir conditions and 
is expressed in barrels per acre-foot of formation. The practice of some 
engineers in using the residual-core oil as the oil in place may apply for 
unflushed cores from shallow low-pressure reservoirs but does not apply 
to cores from high-pressure reservoirs. As indicated in the following equa- 



Subsurface Laboratory Methods 315 

tion, if the porosity and the connate-water data are correct, then the oil in 
place may be readily calculated. O.I. P. = Pg [1 — 8^0) X 7,758 
O.I. P. = Oil in place, barrels per acre-foot at reservoir condi- 
tions, 
Po = Porosity expressed as a decimal. 
Sew — Connate-water saturation expressed as a decimal part of 
the pore space. 
7,758 = Volume of one acre-foot in barrels. 

The oil-in-place equation is based on the assumption that the reser- 
voir pores are completely saturated at reservoir conditions. 

Maximum Recoverable Oil 

The volume of oil that may be recovered from a high-pressure reser- 
voir by an effective water drive and gas expansion when the reservoir 
pressure is maintained at or near the original saturation pressure is termed 
the "maximum recoverable oil." 

The principles upon which this recovery calculation is based in- 
volve the consideration of the flushing and pressure depletion that takes 
place while cutting and recovering the core. As discussed elsewhere, the 
core is subjected to water flood while being cut and to a pressure-depletion 
process while being raised to the surface. These mechanisms occur in 
reverse order in the normal productive life of the formation. 

The following equation is similar to the oil-in-place equation with 
the exception of the correction for the residual oil. The residual oil is con- 
verted into a volume at reservoir conditions and is subtracted from the oil 
in place because there is no means of producing this oil.® 

M.R.= ^'^^~J-~^''"^ X 7,758 
F.V.F. 

M.R. = Maximum recovery, stock-tank barrels per acre-foot of 
formation. 
Po = Porosity, expressed as a decimal part of the formation 

volume. 
Sro — Residual oil at reservoir conditions, expressed as a deci- 
mal part of the pore space. 
Sew — Connate-water saturation, expressed as a decimal part of 
the pore space. 
F.V.F. = Formation-volume factor, decimal expression of the vol- 
ume that one barrel of stock-tank oil occupies at reser- 
voir conditions. 
Although this equation is accepted by most core analysts for calcu- 
lating the maximum recoverable oil from high-pressure reservoirs, the 
writer believes that the values obtained are sometimes much too high. 
The critical factor in this equation is the residual oil, and seldom is it 



' Muskat, M., The Determination of Factors Affecting Reservoir Performance : Am Petroleum Inst. 
Drilling and Production Practice, 1940. 



316 Subsurface Geologic Methods 

possible under field conditions to reduce the residual oil to as low a point 
as that measured in cores in the laboratory. 

It should be noted that the permeability factor does not enter in this 
equation. This factor is definitely reflected in the length of time required 
to recover the oil. Actually it is the capacity (permeability in millidarcys 
times thickness in feet) and the pressure that control the flow rate. The 
higher the capacity, the more rapid the rate of depletion. 

Solution-Gas-Expansion Recovery 

Recovery by solution-gas expansion is usually the initial type of re- 
covery from most reservoirs. The gas in solution in the oil expands when 
a pressure differential is created in the reservoir by producing methods, 
and oil is forced into the well bore. This type of recovery can occur in 
water-drive reservoirs when the rate of withdrawal exceeds the rate of 
water encroachment and a sufficient drop in pressure occurs in the forma- 
tion surrounding the well bore to release the gas in solution. 

Solution-gas-type reservoirs should never be produced at a rate that 
will cause free gas to break out of solution in the formation. This free 
gas restricts the flow of oil and results in high gas-oil-ratio production. 

Reservoirs operating under a solution-gas drive may be expected to 
produce from 10 to 40 percent of the original oil in place. ^ 

Several methods are available for the calculation of gas-expansion 
recoveries. One method that employs the basic core-analysis data has been 
proposed by Johnston ^ and has been used with some eff^ectiveness in fields 
where it was possible to determine the correlation factor. The correlation 
factor used for certain California fields was 0.65. The following equation 
has for its basis the assumption that the gas-filled space in the core at the 
surface is very nearly the same as that which would result from the normal 
pressure depletion of a dissolved-gas reservoir. 

G.E.R. = ^"^^"f;;"'^''"^ X 7 J 58 X C.F. 
r.V.F. 

G.E.R. = Gas-expansion recovery, stock-tank barrels per acre-foot 
of formation. 
Po — Porosity, expressed as a decimal. 
Stw — Total water saturation, expressed as a decimal part of 

the pore space. 
Sro = Residual oil at reservoir conditions, expressed as a deci- 
mal part of the pore space. 
C.F. = Correlation factor, expressed as a decimal. 
The gas-expansion-recovery equation has been used without the corre- 



' Buckley, S. E., and Craze, R. C, The Development and Control of Oil Reservoirs : Am. Petroleum 
Inst., Drilling and Production Practice, 1943. 

"Johnston, Norris, Core Analysis Interpretation : Am. Petroleum Inst., Drilling and Production Prac- 
tice, 1941. 



Subsurface Laboratory Methods 317 

lation factor to calculate the maximum recoverable oil from porous lime- 
stone. 

Gas Reserves 

The dry-gas reserve at reservoir and surface conditions may be cal- 
culated from core analysis, reservoir temperature, and pressure data. 

Vr = Po{l- 5,„,) X 7,758 X 0.005615 
Vr = Gas reserve at reservoir conditions, Mcf. per acre-foot. 
Po — Porosity, expressed as a decimal. 

..-Vr^ (P.+14.7) ^ 520 

Vb— tt ^ TT^ ^ 



Z 14.7 (460+r,) 

V s = Gas reserve at surface conditions, Mcf. per acre-foot at 14.7 

p.s.i. and 60° F. 
V r — Gas reserve at reservoir conditions. 
Pr — Reservoir temperature, degrees F. 
Tr = Reservoir pressure, p.s.i. 
Z = Compressibility factor, expressed as a decimal. 

Recoverable Gas 

The recoverable gas may be calculated to any required residual pres- 
sure from the surface gas reserve. 

RG-y y,^lLZlA±Ml 

R.G. = Maximum recoverable gas to required residual pressure, 
Mcf. per acre-foot. 
Vs = Gas reserve at surface conditions, Mcf. per acre-foot. 
P,- = Final residual reservoir pressure, p.s.i. 
Pf = Final residual reservoir pressure, p.s.i. 

Although this equation is mathematically correct, the calculated re- 
coverable gas estimate is seldom attained. Conservative estimates of the 
recoverable gas may be determined by multiplying the value obtained from 
this equation by 0.60. 

It is apparent from the discussion of hydrocarbon reserves and re- 
coveries that the calculation of these volumes is not merely the substitution 
of figures in an equation. 

Graphic Presentation of Core Data 

The core-log (figs. 122, 126, and 130) is a means of presenting the 
data for visual comparisons. The gas and water-permeability curves, to- 
gether with the porosity curve, are of value in the selection of completion 
intervals and may be used to determine the optimum footage available for 
gas or water injection in pressure-maintenance or secondary-recovery pro- 



318 Subsurface Geologic Methods 

grams. The water-permeability curve will show the portions of the sand 
body that will most readily take water in water flooding and will also 
show where water production will first occur if water encroachment takes 
place. 

The plotting of the impermeable barriers enables the operator to 
make use of natural shutoffs for the control of high gas-oil-ratio and 
bottom-water production. 

The residual-oil plot shows the oil found in the core and is expressed 
in percentage of the pore space. Where gas-condensate or distillate is 
measured, the residual condensate is plotted as oil type (figs. 122 and 
126). 

The production index is based on a relationship between the analyzed 
physical characteristics of the cores and is an expression of the type of 
production to be expected. 

The gravity of the residual oil as determined by core analysis or 
drill-stem tests is presented on the core-log whenever the formation is 
considered oil-productive. 

Summary of Applications of Core Analysis 
Exploration 

The analyses of representative core samples and the intelligent inter- 
pretation of the data are aids in the evaluation of the commercial possi- 
bilities of wildcat wells, edge wells, and field extensions. Core analyses 
together with electric and mechanical well logging make possible corre- 
lation work. 

The determination of the oil gravity and of the productive capacity of 
oil-bearing formations minimizes the possibility of completing a dry hole 
or missing a productive formation. 

Where wells are drilled to total depth without coring, an electric log 
can be run, and the zones that exhibit possibilities of hydrocarbon satura- 
tion may be cored with any one of a number of sidewall-coring devices. 
The analysis of sidewall samples is a highly specialized technique, and 
the interpretation of the core data requires an extensive knowledge of the 
physical characteristics of each formation as represented by full-diameter- 
core data. 

Exploitation 

Core-analysis data may be used with reservoir-fluid-analysis data to 
predict the productive history of a formation and thus predetermine the 
required capacity of the field equipment throughout the producing life 
of the reservoir. Selection of the most effective well-spacing pattern may 
be guided by core analysis. 

Evaluation of oil and gas reservoirs can be made by the use of core 
data, and the probable recoveries can be calculated. 

Core-analysis data are essential for reservoir-performance predictions 



Subsurface Laboratory Methods 



319 



that form the basis for pressure maintenance, repressuring, and secondary- 
recovery programs. Unless complete core data are available for all gas- 
or water-injection wells, it is generally impossible to predict the course 
of the injected fluids. The permeability profile may be used selectively 
to perforate or pack water- or gas-injection wells so that zones of low 
permeability will not be by-passed by the injected fluids. 

Core permeability data may be used to plot isoperm maps and thus 



CORE ENGINEERS 




Reservoir Core Analysis 




SAN ANTONIO. TEXAS 






WELl S 


- 




CORE DATA SUMMARY 


.ONEC.O,V,S,ONNO 


1 














DEPTH. FEET 


1019 - 1Q!\II\ 














PROBABLE PRODUCTION (I) 


Oil 














ANALYZED PRODUCTIVE. 

FEET 


25 - 25 

25 














COKER.COVERV. r. 


100.0 














"ll,darcv"""°"-'^''- 


95U 














CUMULATrVE CAPACITY. 


23.825 
35.7 














sxr/E?fp%°."°Ai'.i>coT 


2770 














SAlEpSf.SS^'S^pli-Rf'sPACE 


32.2 
















35.5 














3a"r?Ti§STaiIcULAtIS 


18 














RESIDUAL OIL GRAVITY. 'API 


20 














^^-St:4iS-o.uu...cro.. 


1.05 














CUSIC FEIT PER BARREL (2) 


150 














BABREL8 PER ACRE FOOT 13) 


2271 














RECOVERABLE OIL. STOCK TANK BARRELS PER ACRE-FOOT 


lxPAnsV5N°^?*^ 


550 














INCREASE BY EFFECTIVE 


725 














r^^rvrAT'lR^O^R^??-,,, 


1275 














*•» REFEH TO REPORT LETTER. 14) APTER PRESSURE REDUCTION PROM 

(5) ORIGINAL RESERVOIR PRESSURE MAINTAINED 


.3) OIL VOLUME AT ORIGINAL RESERVOIR CONDITIONS. ... GAS PHASE RESERVOIR, NO ESTIMATE. 






LUIIVI 


»»0 CONF,Ot~T,AL U.> Of TH. CLI.HT. HO .<.POH.I.,LITY A< TO THt F.ODUCT,V.TY O. PKOP.TA.L.NC. OW ASY lOXI ANALY1.D WILL .. A..uyu> .Y CO.I EXO 


"""• 



Figure 133. Summary of core data, oil reserve, and recoverable oil. This sand has 
a very high productive capacity as measured by dry-gas flow. The sand should 
be cored and completed with oil-base fluids. The low connate-water saturation 
together with the high porosity results in high reserve and recoverable-oil esti- 
mates. 



320 Subsurface Geologic Methods 

make possible the correlation of zones of like permeability in a reservoir. 
The possibility of gas or water channeling can be minimized by using the 
permeability data for selective shooting, plugging, acidizing, or packer 
setting. 

Core analysis is a highly specialized phase of petroleum-reservoir 
engineering. The technique and procedures employed in analyzing cores 
vary with the formation and type of core samples being tested. These 
techniques may be considered somewhat mechanical, but the interpretation 
of core data requires experience. There are many core analysts but few 
qualified core-data interpreters. The use of one set of rules without com- 
mon sense or judgment usually results in bad predictions on the part of 
the interpreter. An attempt has been made to point out the factors beyond 
the control of the interpreter, and due consideration should be given by 
the oil operators for these extenuating factors. 

The practical value of core analysis to the petroleum industry is 
evidenced by its ever-increasing use in the exploration and exploitation 
of gas and oil reservoirs. The cost of this specialized service is small in 
comparison to the present-day cost of drilling a well. To core a well 
without analyzing the cores is as obsolete as drilling the well without 
making an electric-log survey. 

Sometimes hundreds of thousands of dollars are spent in order to 
core but a small fraction of the total depth of the well. Merely smelling, 
tasting, and blowing through these cores can not tell how much oil or 
gas is present and how much can be recovered. Such predictions are the 
work of the qualified core analyst. 

Acknowledgments 

Appreciation is expressed for the encouragement, valuable criticisms, 
and suggestions made by the writer's associates, Robert Caran, C. E. 
Gordon, R. C. Kenney, S. H. Caran, and E. C. Lamon. 

All of the well data presented in this paper were taken from the files 
of Core-Engineers and represent information from actual analyses; the 
company and well names were withheld because it was felt that these wells 
were selected merely as examples, and the data were not intended to be 
used for correlation or exploratory work. 



FLUOROANALYSIS IN PETROLEUM EXPLORATION 
JACK DE MENT 

Luminescence is the emission of light by matter under the influence 
of energy; such a light emission violates the laws of thermal radiation. 
Two types of luminescence are generally recognized: one, called "fluo- 
rescence," lasts only as long as the matter is under the influence of the 
exciting energy; the other, called "phosphorescence," persists after the 
exciting energy has ceased to influence the matter. The fluorescence cor- 



Subsurface Laboratory Methods 321 

responds to the popular designation "glow"; that of phosphorescence to 
"afterglow." 

Luminescent systems are of two kinds, both of which enter into the 
theory and application of fluoroanalysis to petroleum detection and ex- 
ploration. The homogeneous luminescent system consists of matter in 
which all particles are identical, i.e., possessing no impurities and, in the 
case of solids, ideal crystallographic and stoichiometric characteristics. In 
the case of liquids, many highly purified organic compounds very nearly 
conform to the requirements for a homogeneous luminescent system. The 
heterogeneous luminescent system, in contrast to the homogeneous, con- 
tains impurities and/or structural imperfections, and these may enter 
into the luminescence. Solid heterogeneous luminescent systems are il- 
lustrated by the light-emitting materials called "phosphors," which contain 
a trace of activating substance, usually a metallic salt, in solid solution 
in a bulk of relatively inert solvent. Liquid heterogeneous systems are 
illustrated by crude oil, which contains many different organic substances, 
one or more giving rise to the luminescent response to ultraviolet light 
or other forms of exciting energy. Oil in a liquid solvent also comprises 
a heterogeneous system. 

A number of monographs and treatises have been written on lum- 
inescence and kindred subjects, and for a more thorough background of 
fluorochemical methods of geophysical exploration it is recommended 
that these be referred to.^ ^^ Likewise, extensive literature is available 
on that branch of analytic science known as fluorochemical analysis, an 
understanding of which is necessary in petroleum applications.^^ ^^ ^^ A re- 
cent review has been given of fluorescent techniques in petroleum explora- 
tion.14 

Petroleum Fluorochemistry 

The luminescence of petroleum has been recognized since the earliest 
days of antiquity. This property has, in years gone by, been known from 
the fact that crude oil may be of a clear color when taken from the 
earth b"»^ is generally greenish in reflected light and claret-red in trans- 
mitted light. 

TTltrqviolet light, filtered free of visible rays, evokes in crude oils a 
fluorescence that is generally blue, blue-green, or greenish. There may 
be considerable deviation from these colors, however, and oils from dif- 
ferent localities may present typical fluorescent colors. 

The fluorescence of crude petroleum, which is excited by visible light, 
is familiar as "bloom." Ultraviolet light of wave lengths down to 2,000 

° Pringsheim, P., and Vogel, M., Luminescence and Its Practical Applications, New York, Inter- 
science Publishers, 1943. 

■'"' De Ment, Jack, Fluorochemistry (extensive bibliography). New York Chemical Publishing Co., 1945. 

" Radley, J., and Grant, J., Fluorescence Analysis in Uultra-Violet Light, 3d ed.. New York, D. Van 
Nostrand Co., 1939. 

*- Danckwortt, P., Lumineszenz- Analyse im Filtrierten Ultra-violetten Licht, 4th. Au£f., Leipzeg, 
Akad. Verlag., 1940. 

■" Haitinger, M., Die Fluoreszenzanalyse in der Mikrochemie, Vienna and Leipzig, Emil Haim, 1937. 

" De Ment, Jack, Geophysics, voL 12, pp. 72-98, 1947. 



322 Subsurface Geologic Methods 

angstrom units excites petroleum fluorescence, although the so-called 
near-ultraviolet, that approximating 3,650 angstrom units, usually excites 
the brightest response. 

Ultraviolet light can be employed on sands, shales, drill cores, and 
drillings, as well as soil samples, not only to detect petroleum but also to 
reveal the presence of other formations or for detecting economically 
important mineral species. More than 200 minerals and gems fluoresce 
under ultraviolet light and other exciting radiations.^^ 

As an activator in solids, little is actually known about the role 
petroleum plays. In friable soils and earths, it is probable that the oil 
exists in physical combination, e.g., sorptively bonded, with these mater- 
ials. Traces of petroleum may be found disseminated throughout a crystal- 
line mineral, producing a fluorescent effect that may be typical of the 
oil or of the mineral and oil combined. Texas localities yield large crystals 
of calcite containing oil, and this is responsible for a fluorescence independ- 
ent of manganese or rare-earths activation. 

Oil shales provide interesting fluorescences under ultraviolet light. 
Common shales ("blaes") appear a very dark brown in the massive state, 
whereas the color may be absent in the powdered shale. With a kerogen 
shale, a rich chocolate-brown may be observed in both lump and powdered 
forms. Torbanite can be distinguished from cannel by the bright yellow 
streaks on a brown background.^® ^^ ^^ Useful indications are obtained as 
to the origin, preparation, and process of cleaning of Esthonian and Man- 
churian shale oils; individual fractions can be graded according to boiling 
points, all by fluorescent response. ^^ 

There is a good deal of information available on the fluorochemistry 
of substances related to petroleum.-^ -^ Asphalts, coal tars, bitumens, coals, 
and sundry organic minerals not only fluoresce, but the fluorescence can 
often be seen at great dilutions. The origin of such materials can be 
empirically identified by the response; a trace of coal-tar pitch, for ex- 
ample, shows in asphalt by its greenish-blue emission when present at a 
ratio of 1:50,000.22 

Petroleum oils and most refined petroleum products, whether liquid 
or solid, fluoresce at great dilutions in various liquid solvents. The sol- 
vents usually preferred include benzene, hexane, ethyl ether, carbon 
tetrachloride, various straight-chain hydrocarbons, and the like. The pres- 
ence of a nitro group or of chlorine in the solvents generally acts to dim- 
inish the intensity of the fluorescence. 

The detection threshold varies according to the method employed. 



^^De Ment, Jack, Ultra-Violet Prod., Inc., Bull., no. 2, p. 1, 1944; Oil and Gas Jour., vol. 44, 
pp. 75-80. 1945; Handbook of Fluorescent Gems and Minerals, Portland, Ore., Mineralogist Publish- 
ing Co., 1949. 

" Radley, G., and Grant. J., op. cil. 

'' Danckwortt. P., op. cit. 

'^ Haitinger. M.. op. cit. 

"Wittich, M., Brennstoff-Chemie, Heft 19, 1927. 

^^ Pringsheim, P., and Vogel, M., op. cit. 

^ De Ment, Jack, op. cit., 1945. 

22Teuscher, W. Chem. Fabrik, Band 53, 1930; Band 54, 1930. 



Subsurface Laboratory Methods 323 

For unaided-eye observations, one part of oil in 100,000 parts of carbon 
tetrachloride can readily be seen. With electronic or photographic meth- 
ods, the detection threshold may range to the order of parts per hundreds 
of millions. Heavy oils can be detected at a dilution of one part in 200,000 
to 500,000 parts of water, and gasoline only at one part in 20,000 for the 
same solvent.-^ Dispersed in soils, oil concentrations of a few parts in tens 
to hundreds of millions can be detected by photographic methods. 

Methods of Fluoroanalysis 

Every * luminescent substance exhibits certain distinguishing fluoro- 
chemical characteristics. Since luminescence is usually visible light, often 
with varying admixtures of ultraviolet and infrared wave lengths, the 
methods and means of optics serve for an analytic appraisal of the lumin- 
escence. 

This means that luminescent light can be subjected to photometric, 
spectrometric, and spectrophotometric determinations. Likewise, since the 
luminescent system must absorb light, i.e., exciting energy, before an 
emission can take place, the absorption characteristics of that system can 
be studied. With the phenomenologic distinction between fluorescence 
and phosphorescence, it is evident that the lifetime of the luminescence 
may be a distinguishing property of light-emitting systems. 

These purely physical techniques, directed to the luminescence per se, 
reinforce a number of chemical and physiochemical methods of analysis 
which have proved valuable in petroleum work. It is not possible to discuss 
here in detail the relative merits or limitations of each of the fluoroanalytic 
methods, since considerable literature is devoted to that subject, but a 
brief resume is in order so that their application to the specialized field 
of petroleum detection and assay can be more adequately appraised. 

Fluorometry 

The photometry, i.e., the determination of intensity, of fluorescence 
is called "fluorometry." The intensity or brightness to the eye, being de- 
pendent in many instances upon physical and chemical conditions and 
upon the nature of the exciting radiation and its wave length, provides 
valuable information about the kind and concentration of a luminescent 
substance in solution. Fluorescence intensity depends upon several fac- 
tors, including (1) the concentration of the fluorescent substance in solu- 
tion, (2) the intensity and wave length of the exciting light, (3) the pH 
and temperature of the solution, (4) the nature of the solvent, and (5) 
the effect of interfering materials that may be present. 

Electronic fluorometry relies upon the response of a photoelectric 
cell or, in some cases, upon a photon counter tube, under standardized 
conditions, for the measurement of fluorescence intensity. A number of 
these instruments have been designed and constructed for chemical and 



' Halstrik, J., Archiv fiir Hygiene und Bakteriologie. Band 128, pp. 155-168, 1942. 



324 Subsurface Geologic Methods 

biochemical applications, and through their sensitivity fractions of a 
microgram of certain fluorescent substances may be detected. 

Optical photometers have been adapted to fluorometry, such as the 
Duboscq type.-^ 

Fluorography 

The appraisal of fluorescent light by photography is called "fluor- 
ography." Both black-and-white and color methods can be used. Weak 
luminescence is often advantageously detected and measured by the long 
exposures possible with photographic emulsions. An ordinary camera, 
the lense of which is covered with a filter transparent to visible light but 
opaque to ultraviolet light, can be employed. In a fluorograph, only the 
emitted light is recorded and portions of an object that absorb ultraviolet 
light but do not luminesce appear dark, as is true of ultraviolet reflecting 
areas, the light from which is intercepted by filter. Through densitometry 
of the photograph the intensity of the fluorescence may be measured. 

The fluorographic method of fluorometry has been adapated to petro- 
leum detection in soil samples ^^ and is described in more detail below. 

Fluoroadsorption Analysis 

The adsorption of fluorescent materials upon an inert, nonfluorescent 
substance serves both to enhance emission and enable detection of separ- 
ated substances from oft-complex mixtures. 

The fluoroadsorption methods of analysis are either two-dimensional, 
i.e., involving paper, or, three-dimensional, i.e., involving solid columns, 
depending upon the adsorbent. In both cases the aim is to isolate oil 
traces from a large bulk of other material, e.g., soil, rock, or solvent, 
depending upon the method used. 

The sorption phenomenon lays the basis for an extremely delicate 
physicochemical method of analysis, and the sensitivity is greatly en- 
hanced by examination under ultraviolet light. Molecules that ordinarily 
do not fluoresce in the condensed phase, as in liquid or solid states, may 
often emit brightly when dispersed by adsorption. 

While the pioneer work on ordinary capillary analysis was done in 
1829 by Shonbein, it remained for one of his students, Goppelroeder, 
just prior to 1901, to apply fluorochemical methods to the technique. 
Important contributions have been made by numerous investigators;^^ one 
of the best recent reviews is that of Germann.-^ 

When a solid column of adsorbent is used, the solution is drawn 
therethrough, the method being known as "chromatographic analysis," 
although erroneously termed "ultrachromatography" when an ultraviolet- 
light examination is relied upon for an appraisal of the results. Various 
crude oils in fractions can be separated upon a column of alumina, fol- 



2* Koch, W., Nature (London), vol. 154, p. 239, 1944. 

=5 Ferguson, W. B., U.S. Patent 2,356,454, Aug. 22, 1944. 

^^ Neugebauer, H., Die KapillaT-Lumineszenzanalyse, Leipzig, Schwabe, 1933. 

" Germann, F., Colorado Univ. Studies, vol. D, pp. 1-14, 1940. 



Subsurface Laboratory Methods 325 

lowed by elution with different solvents.-^ Qualitative differentiation of 
the crude oils is made by observing the fluorescence of the different frac- 
tions in chloroform solution and the differences in color between the main 
bulk of the solutions. The capillary-strip method is also employed to 
distinguish between crude oils of different origins, as crude and refined 
oils from Digboi and Assam, some artificial crudes, and a chloroform 
extract of a resin. 

Fluorescence Microscopy 

The fluorescence microscope is an instrument which permits ex- 
amination of an object at high or low magnifications under filtered ultra- 
violet light. Fluorescence microscopy may be of the reflected variety of 
the transmitted variety. Actually there is little difference between the 
two, but for opaque specimens the reflected method is generally the best. 

Since petrographic methods play an important role in geochemical 
and geophysical prospecting, fluorescence microscopy would appear to 
have a promising future in this field as an auxiliary tool. The features 
of petrographic sections that may not be revealing by bright-field micro- 
scopy may well be so in ultraviolet light. The fluorescence microscope 
detects oil traces in drill cores, earths, muds, cuttings, water residues, 
and the like.^^ 

Quenching Analysis 

The fluorescence of petroleum is quenched or reduced in color and 
brightness by certain substances. The prudent use of such substances 
makes possible measurement of fluorescence intensity and in certain in- 
stances identification. Thus, nitrobenzene has been used to measure the 
fluorescence, and, by additions of this substance to a sample, the change 
in response appears to have some value for distinguishing oils.^*^ The pres- 
ent writer has found that in complex organic substances, such as essential 
oils, the application of a given quencher to oil samples of different origin 
causes the brightness and color of fluorescence to vary greatly among the 
samples. In addition to nitrobenzene as a quencher, other substances of 
this character include trinitrotoluene, picric acid, and chlorinated hydro- 
carbons. 

Fluorescence Exploration 

The values of fluorescence and ultraviolet light as aids in correlating 
oil sands were first pointed out in 1936 by Melhase.^^ He stated: 

During the past two years the writer [Melhase] has tested oils from many 
of the California fields and from different sands that may occur in these fields 
and it was found that no two samples of oil exhibited the same quality and 
degree of fluorescence unless they were obtained from identical sands. ... It 

^ Mukherjee, N., and Indra, M., Nature (London), vol. 154, pp. 134-145, 1944; Inst. Petroleum 
Jour., vol. 31, pp. 173-178, 1945. 

^ De Ment, Jack, Oil Weekly, vol. 103, pp. 17-19, 1941. 

'" Mukherjee, N., and Indra, M., idem. 

^ Melhase, J., Mineralogist, vol. 4, p. 9. 1936. 



326 Subsurface Geologic Methods 

follows, therefore, that when a well is drilled in any particular field and sam- 
ples from each of the various sands penetrated are tested and classified accord- 
ing to their respective fluorescent qualities, the record thus obtained becomes 
an index for that field. When subsequent wells are drilled the new samples 
may be compared with the index and correlated accordingly. It is necessary, 
of course, that the samples to be tested consist of oil or of sands containing oil. 

The observations of Melhase, while classic, have given way to more 
accurate and standardized, though often empirical, laboratory and field 
methods. The correlations are not limited to shales, but also include any 
earth-bearing oil, either of surface or of subsurface origin. 

The method, it must be emphasized, is neither foolproof nor per- 
fectly reliable. As many of the techniques are empirical, developed in 
the laboratory according to the bents of the technician, there may be con- 
siderable latitude in the extent of dependability of results. The inspection 
of untreated earths with the unaided eye is open to much more error than 
the study of these materials either with instruments or by extraction and 
subsequent objective measurement. Trace amounts of highly fluorescent 
minerals may negate results for an inexperienced worker. Likewise, em- 
phasis should be given the possible untoward effects of air on the sample, 
since some cores, after aging and exposure to subtropical climate, may 
lose their fluorescence. The role of hydrocarbonoxidizing bacteria in the 
destruction of oil traces in rock or earth should not be discounted. 

It has been found that well cores and cuttings, even when small, 
broken, or partly contaminated, fluoresce if they contain liquid hydro- 
carbons.^- In general it is found that a producible sand will give uniform 
fluorescence throughout, but that a sand carrying salt water in addition 
to oil will show a mottled fluorescence. In local areas increases in oil 
saturation and productivity appear to increase fluorescence intensity. When 
properly employed, fluorescence discloses the presence of petroleum un- 
detectable by odor, taste, stain, saturation, analysis, or electric log. 

Fluorologging 

Well-log curves prepared from fluorochemical data provide extensive 
and reliable information.^^ The curves are called "fluorologs" (fig. 134), 
and the data are obtained from cuttings and core samples as customarily 
taken for paleontologic analyses. Core samples are not necessary in the 
procedure which has been developed by Ferguson, but it is desirable to 
have samples of all cores taken. The cuttings samples are usually taken 
at an interval of not more than thirty feet, but samples taken at shorter 
intervals provide more accurate results. Sampling is initiated at the 
surface and continued at regular intervals to the total depth of the hole. 

The assay is made by the photographic method, and fluorescence in- 
tensities are plotted against depth. It is not necessary to rely upon the 
photographic procedure, but it is useful for very weak fluorescences. A 

^- De Ment, Jack, op. cit., 1947. 

^Ferguson, W. B., and Campbell, 0. E., The Fluorographic Method of Petioleum Exploration: 
10 pp., Houston, Texas, Fluorographic Exploration Co., Apr. 24, 1945. 



Subsurface Laboratory Methods 327 

log is prepared from composite samples, which are obtained by mixing 
proportional amounts of the samples from 200 feet of hole. If the fluores- 
cence intensity of a composite sample runs above a given value, a possible 
pay sand is indicated somewhere in the interval covered by the sample. 
The depth and the true value of the showing are established by analyzing 
each of the samples taken within the interval of showing. That part of a 
log based on analyses of individual samples is referred to as "detailed." 
The effectiveness of fluorologging for locating commercial pay horizons 
can be appreciated only from personal experience with the results obtained. 

The over-all characteristics of the fluorolog of a well and the fluoro- 
log of a dry hole are markedly different. A dry hole is characterized by 
low values from the surface to the total depth of the hole. A producer 
that cuts a fault between the surface and the producing horizon often 
shows a fluorolog characteristic of a dry hole above the fault, but a fluoro- 
log typical of a producing well below the fault. 

On a fluorolog of a producing well relatively high values are shown 
at and for some distance below the surface, with a progressive increase 
from the surface to the producing zone. Between pay horizons, in wells 
having multiple producing beds, the curve shows a continuation of high 
values, but it shows a decrease below each sand and a buildup as the next 
producing formation is approached. 

Fluorologging is adapted to small samples, such as side-wall cores, 
which are insufficient in size to be satisfactorily analyzed by core analysis. 
Moreover, data are obtained on sections of a drill hole lost by stuck pipe, 
a blowout, and similar difiiculties before an electric log is run. Fluorologs 
supply information concerning the production possibilities of all strata 
penetrated and afford an independent check on electric logs, core analyses, 
and other data. 

Fluorographic Exploration 

Among the recent developments in the fluorochemical method of lo- 
cating petroleum is the technique of fluorographic exploration, which de- 
pends upon the response of soil samples to ultraviolet light; the samples 
are collected over the area to be explored, and drilling is not required. 

Fluorographic exploration was originated several years prior to 1943 
by Blau,^^ and recently procedural improvements have been directed to 
the assay of soil samples.^^ ^^ 

In fluorographic reconnaissance field work, soil samples are taken at 
stations a quarter of a mile apart on a regular grid pattern. A reconnais- 
sance survey is adequate for most projects, as it establishes approximately 
the outline of a favorable prospect and develops considerable detail. If 
greater detail is desired, the samples may be taken at shorter intervals; a 



" Blau, W., U.S. Patent 2.337,443, Dec. 21, 1943. 

^ Squires, R. M., U.S. Patent 2,451,883. Oct. 19, 1948. 

^' Stevens, N. M., and Squires, R. M., U.S. Patent 2,451,885, Oct. 19, 1948. 



328 



Subsurface Geologic Methods 



H 
UJ 
LlI 



a. 
lij 
Q 



2500 



2600 



2700 



2800 



2900 



3000 



3100 



3200 



3300 



3400 



Figure 134. Section of a fluorolog, showing manner in which fluorescence intensity 
of core sample or the like is plotted against depth at which sample was ob- 
tained. 



Subsurface Laboratory Methods 329 

minimum area of 25 square miles should be covered in a survey, so that 
anomalies can be established. 

Equally satisfactory results have been obtained from samples taken 
in all types of terrain from swamps to sand dunes in all parts of Texas 
and in parts of Louisiana, Mississippi, Kansas, and New Mexico. Soil 
samples taken at different times and under different conditions exhibit 
the same fluorescent characteristics, so that a fluorographic survey can be 
repeated with equivalent results. 

The Blau technique depends upon the fluorescence characteristic of 
petroleum or its derivatives, the employment of soil standards of known 
concentration, and the empirical correlation of soil samples with the 
locality from which they were obtained. The measurement of fluorescence 
intensity is made by photography, and the interpretation is performed by 
an. experienced oil geologist. 

The novel feature of fluorographic exploration is that drilling is not 
required. The soil samples are collected to a depth of several inches from 
the surface. Plant material is carefully excluded, so as to avoid contamina- 
tion. In a given region all samples are taken from uniform depths. 

The soil samples as obtained are placed under ultraviolet light and 
the fluorescence intensity determined by comparison with standards pre- 
pared from nonfluorescent soil containing ten, thirty, forty, fifty, and so 
on parts of oil per million of soil. Visual examination is suitable for 
samples carrying large amounts of oil. 

A fluorographic survey is interpreted by posting the* fluorescence- 
intensity values on the survey grid according to the location from which 
they were taken and by drawing contour lines, called "isofluors," to cor- 
respond to the fluorescence values of the stations. A subsurface accumula- 
tion of oil may show isofluoric enclosure. If structure is present, an iso- 
fluor map may disclose the geologic pattern of that structure. The iso- 
fluor map is effective for locating some faults and minor geologic features, 
and the details may enable a geologist to differentiate between a structural 
and a stratigraphic accumulation. 

The final interpretation or recommendation is empirical, and valid 
data are obtained only after a study of a series of maps embracing both 
the most detailed and the most generalized of information. 

SHALE DENSITY ANALYSIS 
F. WALKER JOHNSON 
Shale density is a criterion of formation evaluation which is some- 
times applicable to subsurface geological problems. Shale compaction, a 
major factor in density variation, has long been recognized by geologists 
as being an important geological process.^'^ Compactio\of shale is the 
result of pressure exerted by the weight of overlying sediments and, in 



^' Hedberg, Hollis D., Gravitational Compaction of Clays and Shales: Am. Journal of Science, Fifth 
Series, vol. 31, no. 184, pp. 241-287, 1936. 



330 Subsurface Geologic Methods 

part, by tectonic movements. Conglomerates, sandstones, limestones, and 
most chemical precipitates show very little reduction in rock volume as 
result of gravitational pressures. However, the fine-grained, argillaceous 
sediments show maximum volume reduction of more than 80 percent. In- 
asmuch as a large percentage of the sediments of the earth's crust are 
composed of clays and shales, their compaction is of special interest to 
the geologist. Reduction of rock volume by compaction is related to the 
weight of overburden and resultant reduction of porosity, and possibly, to 
a certain extent, age of the sediments, and tectonics. Compaction therefore 
results in density increase and porosity decrease. 

Investigation by several workers, notably Athy ^^ and Hedberg,^^ has 
shown the relationship of increased density with greater depth of burial. 

Definitions 

Several terms used in shale-density analysis work require clarifica- 
tion to form a basis for discussion on this subject. Adopting, in general, 
the nomenclature of Hedberg,^^ these may be described as follows: 

Bulk Density (rock density, lump density) is the density of the 
thoroughly dry rock, that is, the rock with pore space free of liquids. Hed- 
berg's samples were weighed in air, coated with paraffin and weighed in 
water. Corrections were made for temperature of water and water con- 
tent of the sample. The water content was determined from the difference 
in weight of the powdered sample before and after thorough drying at a 
temperature of 110 to 120 degrees C. This is also sometimes called "dry 
density," particularly when applied to naturally dried core samples. 

Grain Density (mineral density, absolute density) is the density of 
the constituent particles of a rock, that is, the rock substance free from 
pore space. 

Natural Density is the density of the rock with all pore space filled 
with water which is assumed to be the usual condition found in nature. 
A true natural density of a shale sample is, in most cases, essentially im- 
possible to obtain with complete accuracy as there is a certain amount of 
loss of density due to dissipation of fluid or gases when a core sample is 
brought to the surface in a well. This is particularly true at the greater 
depths, where subsurface pressures are very high. However, a result 
approaching natural density can be obtained with fair accuracy if the 
sample is weighed immediately after extraction from the core barrel. 
Some geologists refer to this as "wet density" inasmuch as the true 
"natural density" can be most nearly obtained from wet core samples 
while they are still saturated with subsurface fluids. 

Apparent Specific Gravity. B. C. Refshauge, in a private report, used 
the term "apparent specific gravity" as the specific gravity of a porous 



^^ Athy, L. F., Density, Porosity and Compaction of Sedimentary Rocks: Am. Assoc. Petroleum 
Geologists Bull., vol. 14, pp. 1-24, 1930. 
3" Hedberg, Hollis D., op.cit., 1936. 
^"Hedberg, Hollis D., op. cit., p. 252, 1936. 



Subsurface Laboratory Methods 331 

body, the pores of which are filled with air. Hov/ever, the apparent specific 
gravity of dried core samples, as used in Refshauge's experiments, does 
not signify oven-dried samples, but those naturally dried in an arid 
climate. Consequently, they may retain small amounts of water. Density 
of such samples will, in most cases, average slightly higher than Hedberg's 
bulk density, but less than his natural density. This term is essentially 
the same as "dry density," a term commonly applied to naturally dried 
core samples. 

Compaction is expressed as the percentage of reduction of rock vol- 
ume. 

Method of Determination of Densities 

The most commonly used method of shale-density determination in 
petroleum work is to weigh the sample in air and then in water. The 
density is equivalent to the weight in air divided by the difference between 
the weight in air and the weight in water. The most satisfactory results 
can be obtained by weighing core samples immediately after extraction 
from the core barrel. In this manner results approaching the "natural 
density" of the sediment are obtained. Samples which readily absorb 
water or show evidence of disintegration from contact with water must be 
coated before weighing. Ambroid cement or collodion have proved to be 
satisfactory coatings. However, old X-ray or photographic films dissolved 
in acetone form an effective coating which may be applied by dipping the 
samples in the solution. Samples chosen should weigh at least 200 grams 
in order to obtain the most accurate results. If large-size samples are used, 
an ordinary laboratory balance may be employed. A very fine wire is 
attached to one side of the balance for the purpose of suspending the 
sample in air and water. A Westphall balance can be used for smaller 
samples. 

It is very important that "pure shale" samples be used for density 
analysis. Every effort should be made to select massive shale free from 
sand, lignite, secondary mineralization, or other material which might 
abnormally affect the increase in density brought about by compaction. 
Experience has shown that very satisfactory results can be obtained if 
large numbers of samples are carefully selected by simple inspection. 

Samples dried in an arid climate and which have been in storage 
in a laboratory for considerable time will generally require coating be- 
fore immersion in water. However, successful results have, in many cases, 
been realized on large, naturally dried samples if the operation is carried 
out very rapidly. The results obtained are those falling within the range 
of apparent specific gravity, mentioned above. A series of laboratory 
checks on samples have shown that they lose their natural fluid content 
with time so that results obtained by direct weighing of samples in labora- 
tories will approach bulk density. (See fig. 135.) 




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Subsurface Laboratory Methods 



333 



Sources of Error 

Sources of error in shale-density determinations fall into two cate- 
gories. These are errors which may be introduced by some inherent con- 
ditions of the sample and those acquired during the actual process of 
weighing the sample. 

The inherent conditions of the sample, which result in variations in 
shale density, may be attributed principally to the effects of weathering 
and mineralization. Leaching near the surface or at unconformities will 
result in a decrease in density. Increases in density are often brought 



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SH 


ALE SAMPLES f 
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OUTH PONCA, THOMAS 


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BLACKWELL FIELDS, OKLAHOMA. 
(after athy,a.a.rg. vol.14 No.i) 






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Figure 136. Relationship of density to depth of burial. (After Athy. Reproduced 
permission Am. Assoc. Petroleum Geologists.) 



about by secondary mineralization. In some cases pyritization has caused 
significant increases in rock density. 

Several sources of error are associated with the weighing of samples. 
Size of sample in some cases is an important factor in this respect. Minor 
errors in weighing of a small sample can have a very important effect 
upon the specific gravity, whereas the small errors on large samples will 
have a relatively minor effect upon the ultimate results. Sensitivity of 
scale used in weighing is another source of error. Water absorption by 
porous samples can, in some cases, bring about serious error in density 
determinations. 

The effect of the suspending cradle or fine wire and the cellulose 



334 



Subsurface Geologic Methods 



coating of the sample may be significant, particularly, in the case of small 
samples. However, by using samples weighing about 200 grams, Ref- 
shauge's work indicates that such errors may be reduced to less than 0.01 
of apparent specific gravity in the average sample measured. 

Application of Results 

Shale-density determinations may be applied to the estimation of 
original depth of burial of sediment and the detection of major uncon- 
formities in the section, as evidence of thrust faulting and as indication of 



-sir 



BULK DENSITY 
NATURAL - 
GRAIN - 




-"""^'.S^ 



m • 

4' * • 



iS^_ 



DEPTM OF OVERBUftPgN IN FEET- P 



Figure 137. Relationship of density to depth of burial in Venezuela. 
(From Hedberg, Am. Jour. Sci) 

the presence of weathered zones. Such data is also of value in geophysical 
work, particularly in the interpretation of gravity-meter surveys. 

Hedberg has published an excellent summary on gravitational com- 
paction of clays and shales in which he presents charts showing the re- 



Subsurface Laboratory Methods 



335 



EL ZAMURO NOI 

WET DENSITIES 

DENSITIES OF CLAYS AND SHALES AVERAGED O/ER 
500 FT INTERVALS .ANDNUMBER OF SAMPLES. 

DENSITIES OF SANDS, INDIVIDUAL SAMPLES 

DENSITIES OF EOCENE SHALES INDIVIDUAL SAMPLES 




-tr»-AVERAGE OF 4 EOCENE SHALES = 2 50 



©-(-AVERAGE 0F488 CLAYS AND SHALES=229 



-AVERAGE DENSITY OF 50 SANDS= 2.12 



EOCENE 



2000 
DEPTH IN FEET 



Figure 138. Average Miocene and Eocene shale densities in Zamuro No. 1, Falcon, 
Venezuela. Note density increase at unconformity indicating that Eocene may 
have been buried much deeper than at present, and that considerable section 
was removed before deposition of Miocene. (Data by C. C. Fritts, Jr.) 



336 



Subsurface Geologic Methods 

MOTATAN NO 1 (VENEZUELA) 




4000 5000 

DEPTH IN FEET 



Figure 139. Average shale densities in Motatan No. 1, Maracaibo Basin, Venezuela. 
Note density decrease at unconformity indicating that Eocene shales had at- 
tained present density before uplift and erosion. It is estimated that about 4,000 
feet of sediments were removed by erosion and that subsequent burial under 
8,100 feet of Miocene sediments had not been sufficient to renew the compactior 
process. (After Refshauge and Skeels.) 




DRY DENSITY - DEPTH CURVE 
OF THE EOCENE SHALE IN THE MARACAIBO BASIN 

PEPARED BY B C SEFSHAUGE 



3000 4000 



I20}0 13000 



Figure 140. Dry densities versus estimated depth of burial in two Eocene wells in 
Maracaibo Basin, Venezuela. Over 500 samples are represented by these curves. 
This interpretation conforms to the generalized regional geological data. (After 
Refshauge and Skeels.) 



Subsurface Laboratory Methods 



337 



lationship of shale density and shale porosity to depth of burial. ^^ D. C. 
"Skeels, in a private report, has accumulated additional data, some of 
which is shown on the accompanying figures. However, some geologists 
believe that age of sediment as well as depth of burial may be a pertinent 
factor. Therefore, arbitrary determinations of depth of burial from 




4000 
DEPTH IN FEET 



Figure 141. Dry densities from well in Colombia. O = shale samples; X = sands and 
sandy shales. (Measurements by W. G. Herlithy; reported by C. H. Acheson. 
After Skeels.) 

straight density determinations should be used with care, particularly if 
the inherent character of the sediment has been effected by secondary 
mineralization. 

Major unconformities can sometimes be distinguished readily by 
shale-density determination through breaks in density curves. Good ex- 
amples have been observed in Western Venezuela (See figs. 138 and 139) 

" Hedberg, Hollis D., op. cit. 



338 



Subsurface Geologic Methods 



where an abrupt break in the density curve marks the unconformity be- 
tween the Miocene and Eocene. 

Thrust faulting may, in some instances, be detected through careful 
shale-density studies. This is particularly true in cases where displace- 
ment due to thrusting involves several thousand feet of section. 

Weathered zones, particularly at major unconformities, can usually 



if 

2.6 


















/ 


25 














IS 

• 


.-"''<" 


^ 


Z* 












^* 








L3 


25 

• 


^y' 


^^<*« 


-^^ 


." 










^2 •* ^ 




23 

• 






AVERA 


GE DENSITIES BY 1000 FT INTERVALS 


21 










COLOMBIA 






AVERAGE SHALE DENSITIES BY lOOOFT. INTERVALS, 
FOR laS SAMPLES FROM 5 WELLS IN COLOMBIA 
LA PUERTA No.l, U-3^ CIMITERRA No. I, 
EAST INFANTAS No.5, WELL 714. 

NUMBER OF SAMPLES, INDICATED FOR EACH AVERAGE. 
SAMPLES DESCRIBED AS 'SANDY SHALE"OR 'LIMEY SHALE" 
WERE NOT INCLUDED IN THE AVERAGES. 
DATA FROM C.H. ACHESON'S MEMORANDA 



9.000 10.000 



Figure 142. Average shale densities from Colombian wells. Note that densities are 
higher than averages for all samples. (After Skeels.) 




Figure 143. Calculated wet density curves. Note that curves A, B, C, D, and / 
agree fairly well. Athy's curve (G) and La Puerta No. 1 (/) fit data much bet- 
ter if curves shifted 2,000 feet and 2,500 feet to right, suggesting greater original 
depth of burial than now indicated. Curves K and H fit poorly, probably due to 
averaging results of several wells. (After Skeels.) 



2 B 










































A 





—a 


J 





• F 
















^ 


^ 






















.^^ 


^ 


















2.3 


O 




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^^ 


p>- 


















o 

V 


-^ 

,^^/ 


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2.2 


























20 


« 


^ ^^_ 














WET D 


INSITY 


VS DEF 


TH 































DEPTH OF BURIAL IN FEET 



Figure 144. Wet density versus depth. Curves U and K eliminated as non-typical. 
Curve G shifted 2,000 feet and / shifted 2,500 feet to right. (After Skeels.) 



340 



Subsurface Geologic Methods 



be distinguished by shale-density determinations. Densities will usually 
be less immediately below the unconformity, especially if the uncon- 
formable surface was exposed to weathering and resulting leaching for 
an appreciable length of time. In some cases, it has been suspected that 





































































__„ .— " 


-^ — 


- 
































aK/ 




/ 
/ 


/ 
















't/ 


f 


y 


/ 
















/ 


/ 


/^ 


f 
















ii 

1 

to 


/ 




/ 

/ 


















/ 

/ 




/ 

/ 
/ 




















/ 




/ 

f 










SH 


^LE [ 


DENSI 


TY 






r 

/ 
/ 










AS A 


FUNCTION OF DEPTH OF BURrAL 


/ 


/ 

V 






















/ 

/ 
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1 
























10 


00 20 


00 30 


00 40 


00 50 


00 60 


00 70 


00 8C 


00 90 


00 !« 


>oo lie 


OO 120 



bEPTH OF BURIAL IN FEET 

Figure 145. Average shale-density curves. (After Skeels.) 



variations in shale densities along fault zones may be associated with 
weathering. 

Some evidence has been observed that generalized correlation by 
shale density may be accomplished in local areas. However, such data 
must be used with care. 

Hedberg ^^ has shown the relationship of decrease of porosity with 

" Hedberg, HoUis D., op. cit., p. 263. 



Subsurface Laboratory Methods 



341 



increasing pressure as being fundamental in compaction studies. There- 
fore, density data may be applied to the estimation of porosity in shales 
(fig. 146). 

Shale density adds another valuable tool to geologic investigation, 
especially when used with other geologic data and interpreted from a 
broad viewpoint. 



GRAPH FOR ESTIMATING WET DENSITY 
AND POROSITY FROM DRY DENSITY DATA 

ASSUMING 

l.MAIN DENSrTY = 2.70 
a. WET DENSITY REFERS TO A ROCK 



DRY DENSITY 



Figure 146. Density-porosity relationship. Wet density = dry density X .615 + 1.04. 
This is a poor substitute for the method of obtaining wet densities at the well. 
(After Skeels, Standard Oil Company of New Jersey.) 



Questions 

1. In what ways has micropaleontology aided the oil industry? 

2. What are Foraminifera, ostracodes, diatoms? 

3. The vertical distribution of microfossils and their relative abundance 
at various stratigraphic positions may be graphically illustrated as 
shown in figures 37 to 39. How are such charts prepared? 

4. What is meant by a "facies log"? 

5. May microfaunal assemblages differ along any given time surface? 
What effect does this have on correlation problems? 

6. What morphological features are treated in describing ostracodes? 

7. What are calcareous algae, and in what type of environment do they 
develop? 

8. What are conodonts, Radiolaria, and otoliths? 

9. Study the flow chart of a medium-sized micropaleontological labora- 
tory given in figure 50, 

10 Should a micropaleontologist in evaluating a stratigraphic section be 



342 Subsurface Geologic Methods 

concerned only with the study of microfaunas? Why? 

11. How is detrital mineralogy helpful in establishing subsurface corre- 
lations? 

12. How are rotary well samples obtained? 

13. What is the difference in quality of a rotary and a cable-tool well 
sample? 

14. What are the chief disadvantages of coring? 

15. What major characteristics of a rock should be observed during 
binocular examination? 

16. Define porosity, permeability, texture, structure. 

17. Hills recommends two principal procedures for describing well cut- 
tings. What are they? 

18. What is meant by "heavy minerals" in sedimentary deposits? Name 
several common heavy minerals. 

19. Briefly outline the procedure for preparing heavy mineral fraction- 
ates? 

20. How is heavy-mineral data used in correlation work? 

21. What is the difference between the "roundness" and "sphericity" of 
a sand grain? 

22. What are the advantages of studying detrital minerals in thin section? 

23. Define "insoluble residue." How are such residues prepared? 

24. What materials constitute the more common "insolubles" ? 

25. Define: dolomold, drusy, euhedral, oolith, granulated, spicular, and 
subhedral. 

26. What are the primary means for differentiating various cherts? 

27. How are insoluble residues used in correlation work? 

28. A method of plotting insoluble-residue data is shown in figures 58 
and 59. Study. 

29. What procedure is followed in preparing a thin section of friable 
material ? 

30. Define petrofabrics. 

31. Give a map symbol for: plunging syncline, reverse fault, concealed 
anticline, overturned anticline. 

32. What procedure is followed in collecting a sample in the field for 
petrofabric study? 

33. What are the applications of petrofabric studies? 

34. How may size analysis be applied in stratigraphic investigations? 

35. What is a histogram, simple-frequency curve, and cumulative-fre- 
quency curve? 

36. How is the coefficient of sorting of a sediment determined, and what 
is the significance of this value? 

37. What staining methods are used for distinguishing calcite from 

38. Outline the procedure for preparing a clay sample for stain anlysis. 



Subsurface Laboratory Methods 343 

39. How is kaolinite distinguished from montmorillonite by use of stains? 

40. What is the importance of knowing whether a montmorillonite clay 
or kaolinite clay is involved in construction problems? 

41. What factors control the shape of sedimentary grains and fragments? 

42. What is the relationship between porosity, grain size and perme- 
ability? 

43. What magnifications are involved in electron-microscope studies? 

44. What are the possible uses of the electron microscope in correlation 
work? 

45. Is there any relationship between subsurface lithologic units and their 
water characteristics? 

46. How are ground waters classified? 

47. Study the graphic method of representing ground-water data in figure 
120. 

48. Compare the composition of Lower Cretaceous waters of the Rocky 
Mountain Region given on table 11. 

49. What are the advantages and limitations of the X-ray diffraction 
analysis? 

50. How is the sample prepared for an X-ray mineral determination? 

51. Study the X-ray pattern given on plates 7 and 8. 

52. How may X-ray data be applied to stratigraphic investigations? 

53. What is the purpose of core analysis? 

54. What information is given on a core-log? 

55. How should one sample a core from which a detailed analysis is to 
be made? 

56. What factors control the amount of residual oil in a core sample? 

57. What is meant by "connate water" and what methods are applied for 
calculating the connate-water saturation of sand formations? 

58. How are core data used for interpreting probable production? 

59. What physical characteristics are associated with permeable, high- 
pressure gas sands? 

60. What is meant by "bleeding cores"? 

61. What are the applications of core analysis? 

62. What is the general principle and procedure of differential-thermal 
analysis? Could such a technique be applied to certain stratigraphic 
problems? How? 

63. What rock types are best suited for differential thermal studies? 

64. What is the value of shale-density data? 



CHAPTER 5 

SUBSURFACE LOGGING METHODS 

SAMPLING AND EXAMINATION OF WELL CUTTINGS 
JOHN M. HILLS 

Although the techniques described in this paper have been learned 
during 14 years of subsurface work, the writer does not wish to give the 
impression that they are original with him. They are rather the results of 
many years' experience of hundreds of geologists engaged in cuttings sam- 
ple work, which are summarized especially for the benefit of those enter- 
ing the profession or setting up subsurface departments in other areas. 

Well cuttings are the source of most subsurface data obtained in the 
Mid-Continent and Permian Basin areas of the United States. The collection 
and examination of samples of these well cuttings are highly organized 
and important techniques. In fact, in most holes in this province, well 
cuttings are the only reliable source of data concerning the formations 
penetrated. Logs made by experienced cable-tool drillers are very useful, 
but under present conditions these are rare. It is usual to find the driller's 
log made by an inexperienced or uninterested rotary driller. Such a log 
gives only a general idea of the formations drilled, especially in areas 
where there is a distinct formation change between wells. 

Of all the methods of obtaining information concerning rocks cut by 
the drill, cores are probably the most reliable. In soft unconsolidated 
formations these cores can be taken rapidly and with comparatively little 
expense, by means of a wire-line core barrel. However, in the Mid-Con- 
tinent and Permian Basin areas most of the rocks are well lithified, and 
many are extremely hard. For this reason wire-line core bits wear out 
rapidly, and cores are usually obtained by use of conventional core barrels 
which are capable of cutting a maximum of 20 feet at one time. Naturally 
this makes complete coring extremely expensive, especially in deep holes. 
Recently a new technique of coring with diamond-studded core heads has 
been developed which may lead to much more extensive coring. It is 
possible that in the future as much as 100 feet of core may be taken at 
one time by means of a diamond bit without coming out of the hole. This 
may render practicable coring of complete sections of deep holes. 

The writer is especially indebted to W. D. Anderson, who taught 
him the fundamentals of sample examination; to William Y. Penn for the 
pictures of porosity; to E. Russell Lloyd, who suggested the writing of 
this paper and has been generous with help and suggestions; and to John 
Emery Adams and W. W. West, who kindly read and criticized the manu- 
script. 



Subsurface Logging Methods 345 

Electrical Logs and Radioactivity Logs 

Electrical-resistivity and self-potential logs have almost supplanted 
cutting samples and cores in areas where the stratigraphic section is shale 
and sand and where salt beds do not contaminate the drilling mud. How- 
ever, in areas where the section is composed of several kinds of rock, 
especially where limestone and anhydrite are abundant, electrical logs 
must be used with sample logs since the lithologic variety and high resist- 
ance of the rocks introduce many unknowns into the interpretation of the 
log. Thus, it is impossible to solve the problem without a sample log 
which will enable one to eliminate many of the possible solutions. 

In limestone reservoirs it is very difficult to recognize the fluid con- 
tent of the reservoir rock by use of electrical logs without additional in- 
formation from testing the well, because the high resistivity of the lime- 
stone and the sulphur water commonly present mask the resistivity effects 
of the other fluids in the formation. However, electrical logs are useful 
for correlation in local limestone reservoirs where the section is well con- 
trolled by logs from sample cuttings. One fact that should be remembered 
in using electrical logs is that rock-salt beds cut by the bit cause a salty 
mud with a very low electrical resistance that results in a featureless self- 
potential curve that is useless in making correlations. 

Radioactivity logs are also useful as an auxiliary to sample logs. 
Here, also, a wide variety of rock and fluids in the stratigraphic column 
results in several possible interpretations for the curve, and a sample log 
is necessary for the correct solution. Radioactivity logs are sometimes 
very useful in logging old wells that have already been cased off or wells 
where mechanical difficulties have prevented taking a representative set 
of cuttings. Radioactivity neutron logs are probably more useful than 
resistivity logs in indicating a fluid zone, but it is not possible from the 
neutron log alone to determine whether the fluid content of the formation 
is oil or water. 

Thus, in areas where consolidated formations are encountered, exam- 
ination of well-cutting samples is generally the easiest and least expensive 
method of detecting changes in the formation and determining the strati- 
graphic section penetrated by any well. Much subsurface geologic work 
depends on the collection of representative samples of the formation 
penetrated, describing these samples accurately, and plotting the descrip- 
tion so that the sections in diff"erent wells may be correlated. 

Cable-Tool Samples 

The collection of cuttings samples from cable-tool wells presents com- 
paratively few difficulties. The samples should be collected from the first 
bailer after each run of the bit, in a bucket hung at the end of the dump 
box. These samples should be washed enough to carry off all mud. They 
should be put in cloth sacks, then labeled with the name of the company. 



346 Subsurface Geologic Methods 

name of the farm, number of the well location, and depth of sample. 
The hole is, of course, bailed clean each run of the bit, and only occasion- 
ally are cavings a problem in consolidated formations. 

In drilling bentonitic shales, quicksands, or conglomerate, the hole 
ordinarily caves readily so that the samples are not entirely reliable. When 
high-pressure gas is encountered, the hole is commonly filled with water 
or light mud to control the gas. This ordinarily results in samples being 
finely ground and much material being washed off the upper parts of the 
hole, an action which contaminates the samples. The same condition re- 
sults from drilling ahead in a hole full of water coming from the forma- 
tion. Generally, under these conditions, pipe is run before much hole is 
made so that the number of poor samples is comparatively small. Where 
wire line or tools have been lost in the hole and must be drilled up, some 
samples contain large amounts of iron. This can be removed by means of 
a magnet and the residue of the sample examined. Surface rock and other 
materials sometimes are thrown into the hole in attempting to straighten 
it. 

The problem of checking the depth at which samples are taken is not 
critical with cable tools, since the depth is checked by sand line on the 
bailer after each run. However, important datum beds in the section 
should be checked for depth by stringing in the sand line or by running 
a steel measuring line. Cable-tool samples differ from rotary samples 
by the general flaky character of the harder formations and by the polish- 
ing and rounding of many cuttings from attrition due to turbulence set up 
by the bit, and the irregular intervals at which they are taken, since the 
length of bit runs are determined by the character of the formation and 
mechanical factors. 

Rotary Samples 

The collection of rotary samples presents many more difficulties than 
the collection of cable-tool samples and for many years it was considered 
impossible to obtain reliable samples by this method of drilling. It is 
still difficult to obtain representative samples from rotary holes in uncon- 
solidated formations, but a technique has been worked out whereby rep- 
resentative samples can be obtained in consolidated formations. These 
samples are taken from the returning fluid stream at regular intervals. The 
sample interval is usually 5 or 10 feet, but may be as small as 1 foot or 
as large as 30 feet. The chief problems in collecting samples of rotary 
cuttings are to prevent contamination with upper beds, prevent powdering 
of the sample, prevent loss of the sample, prevent elutriation, obtain 
correct depth measurement, and wash and properly dry the samples. 

Contamination from Upper Beds 

Contamination from upper beds is the chief problem of collecting 
rotary samples. In the early days of rotary drilling before mud building 
was fully understood, the walls of the hole were poorly plastered and un- 



Subsurface Logging Methods 347 

stable. Thus, the samples contained large amounts of extraneous material. 
The practice of considering any new material appearing in the samples as 
composing the entire content of the formation drilled then became general 
and any other material was believed to be cavings. At present, however, 
the treatment of rotary muds has advanced so that usually a properly 
mudded rotary hole caves very little and the percentage content of the 
sample may be taken as representative of the types of rock in the interval 
covered. This enables the geologist to follow very closely the lateral grada- 
tions in the section. 

It should be emphasized that the geologist must work with the drilling 
superintendent to insure the maintenance of proper mud in the hole not 
only for the sake of drilling progress but also for the securing of repre- 
sentative cuttings samples. The mud must have a gel strength and viscosity 
sufficient to bring the cuttings to the surface without recirculation and 
regrinding. It must be also capable of cushioning the impact of the drill 
pipe against the walls so as to prevent powdering of the samples. Once 
on the surface, the mud should be run through pits sufficiently large to 
insure settling of all the cuttings so that they will not recirculate. A shale 
shaker will insure complete separation of the larger cuttings and the mud. 

Powdering of Cuttings 

The powdering of the sample to a size too small to be examined 
effectively under the binocular microscope is largely due to poor mud 
which allows regrinding of the sample by the bit and powdering by 
whipping of the drill pipe against the walls of the hole. When extraordi- 
narily large cuttings are needed for analysis of porosity and permeability 
or for other purposes, it is helpful to circulate in reverse of the usual 
manner, that is, to pump the mud down between the casing and drill pipe 
and up through the drill pipe. This process results in higher mud veloci- 
ties returning through the drill pipe which brings larger cuttings from the 
bottom as soon as they are chipped off by the teeth of the rock bit. Some 
of these cuttings are 4 to f inch across. 

Reverse circulation is especially useful when drilling into low-pres- 
sure formations, by using oil as drilling fluid, as it prevents any of the fine 
oil-borne cuttings from plugging the pores of the formation. In regular 
circulation with oil, the low viscosity of the oil will not carry out the 
cuttings, and they are commonly reground to a fine putty-like mass which 
is useless for examination and has a plugging effect on low-pressure pays. 
Of course, reverse circulation is not necessary in high-pressure gas or 
oil pays since the high pressures tend to increase the velocity of the cir- 
culating fluids and carry the cuttings out of the hole. The advantages of 
using a water-free drilling fluid in low-pressure oil pays are combined 
with the advantages of a high-viscosity mud in carrying out the cuttings 
and cleaning the formation in oil-base muds recently developed. With 
these muds it is not necessary to use reverse circulation to obtain cuttings 



348 Subsurface Geologic Methods 

large enough for visual examination. However, since methods have been 
recently developed for determining porosity and permeability from extra- 
large cuttings obtained by reverse circulation, this method undoubtedly 
still will be used to some extent in pay sections. 

Loss OF Cuttings 

Loss of cuttings samples is due to two chief causes: lost circulation, 
and blow-outs. Both of these are primarily mud problems. Lost circula- 
tion is caused either by too high water loss in the mud or by excessively 
heavy mud which overcomes the formation pressure of a porous or frac- 
tured bed so that the mud enters the pores or fractures. The cuttings are 
then carried into the porous beds and may not ever be recovered unless the 
well is completed in this zone and the cuttings come out later with the 
oil. Blow-outs are caused by unexpectedly high formation pressures or 
by carelessness in handling the mud. The cuttings are blown out of the 
hole and not recovered. 

Elutriation 

Elutriation or separation of the coarse from the finer part of the 
sample by the upward movement of the circulating fluid is due to the 
use of mud with low viscosity and gel strength. In good mud the cuttings 
are held in suspension and there is little change in the relative position. 

Methods of Obtaining Correct Depths 

Obtaining proper depth measurement for rotary cuttings is another 
major problem. The depth of the well must be checked often either by 
steel measuring line or by measuring the drill pipe under tension. Atten- 
tion must be given constantly to see that the crew catches the samples at 
^ the proper intervals and that they do not anticipate the sample by filling 

several sacks at one time. 

A great help in assuring correct sample depth is to have the driller 
keep a record of the drilling time on a form, such as table 18. Each 
interval, 1, 5, or 10 feet, depending on the importance of the section, is 
shown on this sheet with the time drilling began, the time it ended, the 
time taken out for mechanical work, and the net time drilling. These 
intervals are usually marked on the kelly with grease or chalk and 
checked by the pipe measurement each time the kelly is drilled down 
to the derrick floor. In addition to assuring the correct depth of the 
samples, the drilling time gives a very valuable clue to the nature of 
formations penetrated as the time of drilling has a very close correlation 
with the lithologic character of the formation. This drilling time also 
may be taken with a mechanical device. 

In pay sections where the exact measurements are very important 
or in deep holes where tlie cuttings take a long time to come to the sur- 
face, samples should be labelled with the depth at which they are actually 



Subsurface Logging Methods 



349 



cut rather than the depth of the well at the time they come to surface. 
This measurement is accomplished by placing some easily identifiable 
substance, such as rice or corn in the drill pipe at the derrick floor when 
making a connection, and measuring the time required to bring the sub- 
stance around to the shale shaker or return pipe. If regular circulation is 
being used, it can then be calculated from the pump pressure and volume 
handled how long the mud requires to go from the derrick floor through 
the drill pipe to the bit, and this subtracted from the total return time gives 
the time necessary for the samples to come from the bit to the surface. 

TABLE 18 
Example of Drilling-Time Form 



Company No 

Location County.. 



Farm. 

State... 



Mud 



Weight on drill pipe Mi 


id weight 


R.P.T 


^ Viscosity , 




Depth 


Drilling Time 


Actual 
Time 




From 


To 


Bepan 


Ended 


Time Out — Remarks 




































' 











































































Actual time is time spent in drilling. Shut-down time, round trips, changing bit, re- 
pairs, etc., also condition and type of bit should be noted under "remarks." 



Unless it is known that the hole is in extraordinarily good shape with no 
washed-out places, it is not satisfactory to calculate the return time from 
the bit by mud volume and velocity, because some eddying and consequent 
lowering of the mud velocity takes place in all washed-out places. If it is 
not possible to determine the sample return time from experimental meth- 
ods of calculation, a rule of thumb is that under ordinary mud pressure 
with 7- or 8-inch hole, it will take cuttings about 10 minutes per 1,000 
feet to return from bottom. 

After the return time of the sample is determined or estimated, each 
sample should be caught that length of time after the appropriate depth 
mark on the kelly reaches the rotary table. An easy way to do this is to 
place on the drilling-time sheet the time each sample should be caught. 
This is done by adding the return time in minutes to the time at which 
the given mark is down to the rotary table. This method will eliminate 



350 Subsurface Geologic Methods 

lag in the samples and should make the sample log correlate very closely 
with the drilling time and with the electrical log. 

Catching Samples 

There are many ways to catch representative samples from the re- 
turning mud stream. The rotary shale shaker of the Thompson type has 
become very common in recent years. This is a large cylindrical screen 
through which the returning mud stream passes. The screen is turned 
by a "water wheel moved by the mud stream. Attached to the large screen 
is a much smaller screen with fine mesh through which a portion of 
the main mud stream is diverted, off which comes a small portion of the 
cuttings which is collected in a box at the end of the screen for visual 
examination. This type of screen has the advantage that it requires 
no outside power to operate it, and it catches a representative sample of 
the cuttings without any further attention from the operator or crew. 
However, unless a very fine screen is used on the sample catcher, it will 
not catch all the fine sands. In some formations the mud will not wash 
out of the cuttings without use of an excessive amount of water which 
renders it unsuitable for use with low water loss muds. 

Another important type of screen commonly used is the vibratory 
shaker. In this device the mud stream passes across a vibrating screen. 
The mud passes through the screen into the pits while the cuttings are 
vibrating off into another receptacle. In consolidated formations good 
samples can be taken by placing a narrow box under a part of the end 
of the vibrating screen so that a representative portion will fall into this 
box. However, this screen is open to the same objection as the Thompson 
machine in formations consisting of fine sands, since the fine sand tends 
to pass through the screen and not be caught in the sample. 

The simplest and possibly most reliable sample-catching device 
(fig. 147) consists of a small-diameter nipple welded to the bottom of 
the mud-return line with a l|-inch or 2-inch line running to a box 
1 foot by 1 foot by 3 feet, with a removable gate about 6 inches high 
in the end. In this box a representative portion of the cuttings is collected 
and may be shoveled out at the appropriate time into a bucket and the 
box cleaned by removing the end gate and letting the mud stream 
carry out the remaining cuttings. When these are washed away, the 
gate is replaced and the collection of the next sample is begun. By 
this means a representative portion of both the fine and coarse parts of 
the cuttings is obtained. Sometimes such a box is set in the course of 
the main mud stream, but so many cuttings accumulate that the space 
behind the gate is filled with cuttings before drilling of the sample interval 
is completed so that the last part of the interval is not represented by the 
cuttings in the box. 

After the samples are caught, they must be washed properly. This 
can be done in a bucket by filling it partly full of water and stirring the 
samples vigorously, letting the clean part settle and decanting the fluid sev- 



t^N^-^-^ ' t 



2s 

55 



CO 






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II 



noi 



z 

o 

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UJ 


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352 Subsurface Geologic Methods 

eral times until the water is clear and the sample is ready for sacking and 
labelling. Sometimes the sample is washed in a box with a fine-screen 
bottom. This hastens the washing process, but there is some danger of 
losing the fine sand from the sample. When drilling with oil or oil-base 
mud, one must wash the sample entirely free of drilling fluid before it 
is dried. This can be accomplished by using hot water, although some 
prefer washing with kerosene or gasoline before washing with water. One 
need have no fear of washing the indigenous oil stain and saturation 
from the sample as any ordinary washing will only remove the drilling 
oil and will not remove the natural oil from the formation. 

After washing, samples are commonly dried artificially. However, 
in samples showing stain and saturation this must be done with extreme 
caution as overheating will blacken the natural-oil stain and mask the 
porosity as well as render it difficult to tell whether the formation fluid 
is oil, gas, or water. In extreme cases of overheating, red shales may be 
oxidized to a black color. If samples from wells drilled with oil-base, 
drilling fluid are not washed clean before drying, it is impossible after- 
ward to remove the drilling oil even by boiling the sample. In some cases 
where samples are to be used for paleontologic examination, any shale in 
the samples is digested with 'boiling water and soda to leave the fossils 
free. It is probably better when there is no necessity for haste in exam- 
ination of the samples to let them dry naturally in the air rather than to 
dry them artificially. 

After the samples are sacked and transported to the laboratory, it is 
commonly necessary to divide them into smaller portions for different 
types of examination or for examination by diff^erent individuals. This 
should not be done by merely pouring a little off the top of the sack as 
ordinarily there will be a few large cavings on the top of the sack. These 
should be raked off and discarded and the remainder of the sack mixed 
so every portion will be representative. All portions or "cuts" of the sam- 
ples should be large enough to be representative. Generally a tablespoon- 
ful should be the minimum. 

Examination of Cuttings Samples 

If the samples are properly prepared as outlined in the preceding 
section, they may be examined without further preparation. However, 
samples may not be washed properly on the rig and they must be re- 
washed before examination. This is most conveniently done in small sauce 
pans. The samples are agitated in water and the fine mud decanted; this 
process is repeated until the water is clear. Machines have been invented 
to help in this operation, but they are economical only where large num- 
bers of samples must be washed. Drying the samples should be done as 
slowly as possible so as not to overheat and burn the sample. If a sample 
is only slightly dusty it may be prepared for examination by shaking 
over a screen of 100 mesh or finer. Some samples contain cavings which 
are ordinarily larger than the cuttings. These can be scraped off the top 



Subsurface Logging Methods 353 

after agitation, and the residue of the sample examined. The same process 
helps to remove gel flake and other plugging materials added to the mud 
to control lost circulation. Iron particles in the samples can be removed 
by a magnet. 

Samples may be examined while covered with water. This method 
brings out by diff"erential refraction some of the qualities of the sample 
such as oolitic structures and anhydrite crystals which may be overlooked 
in dry samples. When examining the sample wet, it is not necessary to 
have them so clean as when examining them dry since a certain amount 
of washing may be done in the water in which the sample is examined. 
Dry examination is more convenient since it is not necessary to have a 
number of petri trays or watch glasses with which to examine the samples 
in water. 

Cuttings samples ordinarily are examined under a binocular micro- 
scope of low power. The exact power used varies with the individual 
geologist and with the nature of the samples. The power should be high 
enough to see the essentia) structure and texture of the cuttings. Porosity 
coarse enough to have a permeability sufficient to make commercial oil 
production should be seen easily. Oolites, inclusions, and other sedimen- 
tary features should be clearly visible also. On the other hand, the micro- 
scope power should be low enough to permit examination of large num- 
bers of samples without eye strain and should permit a field of view 
large enough for estimating percentages of the various constituents accu- 
rately with only a few changes in position of the sample. The powers 
that meet these requirements vary between 12 and 24. Higher powers 
are used for special purposes such as the description of minor features 
of microscopic fossils. For general use, however, in the oil industry it is 
not desirable to have a power higher than 24, since a very high power may 
give false impressions about the permeability and porosity of cuttings. 

The illumination of the microscope field is important. In the past, 
small incandescent lights which may be focused to give intense illumina- 
tion have been used widely. However, with the development of the fluores- 
cent lighting tube, it has been found that the white light given by this 
tube is much superior to the yellowish light of the incandescent bulb. 
This is especially true when searching for oil stain in the cuttings, since 
the yellowish incandescent light commonly masks the light-brown oil stain. 
Usually it is not practicable to focus the fluorescent tube so as to give 
intense light for high-power work. However, for the usual low-power 
examination, the fluorescent tube is very satisfactory. In examining sam- 
ples for oil stain and saturation it may be desirable to use an ultraviolet 
light. 

In examining well cuttings samples the geologist should endeavor to 
estimate as closely as possible the proportion of each rock type in the 
sample and to describe the lithologic characteristics of the rock that are 
essential to the correlation of the beds. In commercial work it is not 



354 Subsurface Geologic Methods 

practicable to give the analytical detail that may be desirable in purely 
scientific research. The geologist, nevertheless, should endeavor to make 
the description as complete as possible in the time available. Necessarily, 
the description should be more detailed on wildcat wells than on field 
wells and much more detailed in the pay sections than in the upper sec- 
tions. In field wells it is permissible to omit descriptions of long sections 
of comparatively insignificant beds between key beds. The geologist 
should remember that it is much better to have too full a description than 
too meager a description. Experience has shown that full descriptions 
have saved much time and money, whereas meager descriptions have re- 
sulted in redescription of the cuttings several times. Nevertheless, some 
redescription can not be avoided; therefore, samples should be carefully 
preserved and indexed so that they will be available for future study. 

There are two principal ways of describing samples. The first of 
these is the interpretative system, in which the geologist picks out the 
■ cuttings which he believes to be representative of the formation penetrated 
and describes the entire sample as composed of this rock. The rest of 
the sample is assumed to be cavings. This kind of description brings out 
formational changes and is of greatest value in areas where the various 
formations are of wide extent and relatively constant character, as in 
the Paleozoic of the Mid-Continent region. In areas of rapid lateral 
gradation in the lithologic character of formations, as in the Permian Basin 
of west Texas, this method results in masking of lateral variations and 
misinterpretation of the nature of the stratigraphic column. This, of 
course, results in miscorrelation of the well logs. 

In regions of pronounced lateral gradation it has been found that a 
second method of sample description is most satisfactory. This is the 
percentage description, where the geologist describes all material in the 
sample, disregarding obvious foreign substances and cavings. This sys- 
tem, though making it difficult to determine formational boundaries from 
the sample log, shows the gradations of the beds and often enables one 
to trace a horizon through different sedimentary facies. 

Percentage of various constituents is estimated by the eye. It has 
been found that experienced sample examiners, using good samples, will 
agree very closely on the percentages of constituents. However, with 
poor samples where some judgment must be exercised in disregarding 
obvious cavings, the value of the description is dependent on the judg- 
ment of the geologist who examines the samples. Mechanical counters or 
calculators, such as the integrating stage, have not been found to give 
enough additional accuracy to be worthwhile in ordinary work. Experi- 
enced sample examiners can describe 100 to 300 samples in an 8-hour day, 
according to the nature of the cuttings.^ 

The descriptions may be written out or plotted directly on a log 
strip. For many purposes, it is desirable to make written descriptions, 

^ A full description of sample examination as applied to stratigraphic work is given by L. H. Lukert, 
Oil and Gas Jour., pp. 49-51, June 1937. 



Subsurface Logging Methods 



355 



TABLE 19 

Example of Sample-Description Form 

Operator No Lease County 

Block 
Section League Survey 

Labor Twp Range Footage 

Elevation T.D Date Examined by.. 



Depth 


Lime OT Dolomite 


Anhy. 






Non Red 
Shale 


Red 

Shale 


Red 
ss. 


Gray 
ss. 




Chen 


From 


To 




% 


Lith. 






% 


Lith. 





































































































































































































































































































since the logs can be plotted in several places at the same time and a 
record is kept which is not subject to destructive wear as is the plotted 
log which is used constantly in the field and laboratory. A type of de- 
scription form is included (table 19) . In addition to the constituents 
shown on this form, others may be specified according to the nature of 
stratigraphic sections in the area worked. Pyrite and differently colored 
shales will be the most likely additions. In areas where sand production 
is important, separate columns for porosity and saturation in sandstone 
may be desirable. 

Color of the rock fragments in cuttings samples is a very important 
attribute. At present there is a wide variation in the descriptions of color 
by subsurface geologists. The same rock may be described as tan by one 
and brown or gray by another. It is desirable in any organization attempt- 
ing to start sample examination work to standardize some color scheme, 
possibly that being developed by the inter-society committee.^ 

Size of particles in dolomites and limestones is another matter on 
which subsurface geologists vary. The following table gives the tentative 
scale of crystal sizes which was developed by the West Texas Geological 
Society several years ago. It would be to the advantage of anyone under- 
taking sample description for the first time to adopt a scale with appro- 
priate abbreviations for the sample-description sheets.^ 

^ DeFord, R. K., Rock Color Chart for Field Geologists: Am. Assoc. Petroleum Geologists Bull., 
vol. 31, no. 10, pp. 1903-1904, 1947. 

^ A more elaborate scale is advocated by Ronald K. DeFord, Grain Size in Carbonate Rocks: Am. 
Assoc. Petroleum Geologists Bull., vol. 30, no. 10, pp. 1921-1928, 1946. 



356 



Subsurface Geologic Methods 



TABLE 20 

Tentative Scale of Crystal Sizes in Dolomites and Limestones 



Crystal Diameter 

Invisible 

Less than 10 mm. 

Less than 0.02 mm. 

0.02 to 0.1 mm. 

0.1 to 2.0 mm. 

2.0 to 10 mm. 



Descriptive Adjectives 

1. Mat 

2. Microcrystalline 

a. Cryptocrystalline 

b. Finely crystalline 

c. Mediocrystalline 

d. Coarsely crystalline 

3. Megacrystalline 

Definitions 
Mat. — Compact, exceptionally homogeneous; having a dull but even surface under low binocular micro- 
scope; resembling limestones used in lithography; as, a mat limestone, a mat dolomite. 
Microcrystalline. — Having crystals less than 10 m.-n. long; having crystals small enough to be viewed under 
low-power binocular microscope. 

Cryptocrystalline.— \adi&Xiacl\y crystalline; showing very small, indistinct crystal faces; composed of 
crystals too small to be measured under low-power binocular microscope. 

Megacrystalline. — Having crystals 10 mm. or more in length; having crystals too large to be readily dis- 
cernible under low-power binocular microscope. 

Porosity, Permeability, and Oil Stain 

Description of the porosity, permeability, and oil stain of cuttings 
samples is one of the most important parts of the sample examiner's work. 
In sandstone, the porosity is determined by the size of the grain, the 
sorting, and the amount of the cement present. The grain size should be 
described by some standard scale such as Wentworth's ^ or Alling's.^ 




Figure 148. Isolated pin-point porosity. Reverse circulated cuttings. x5. Penrose's 
University 2, sec. 3, blk. 10, University lands, Andrews County, Texas; 4410- 
4415 feet. 



* Wentworth, C. K., A Scale of Grade and Cl-ass Terms for Clastic Sediments: Jour. GeoL, vol. 30, 
pp. 377-392, 1922. 

"Ailing, H. L., A Metric Grade Scale for Sedimentary Rocks: Jour. Geol., vol. 51, pp. 259-269, 1943. 



Subsurface Logging Methods 



357 




Figure 149. Granular dolomite with good intermediate porosity. Reverse circulated 
cuttings. xlO. Penrose's University 2, sec. 3, blk. 10, University lands, Andrews 
County, Texas; 4465-4470 feet. 




Figure 150. Leached oolitic porosity in dolomite. x5. Champlin's University 1-D, sec. 
26, blk. 13, University lands, Andrews County, Texas; 8030-8060 feet. 



358 Subsurface Geologic Methods 

The sorting is dependent on the proportion of fine minerals present, such 
as silt or shale, or the proportion of large grains, such as large frosted 
quartz grains commonly found in the Whitehorse section of the Permian 
Basin. The amount of cement also largely determines the porosity which 
is at a maximum in free unconsolidated sands and minimum in quartzite. 
The permeability is largely determined in the same manner as porosity, 
but here the size of grain and the sorting are most important. 

Oil stain and odor are very important and much experience on the 
part of the sample examiner is required to estimate these accurately. 
Dry gas will not stain sandstone, but wet gas may show a very light tan 
stain. Oil is commonly darker, but very high-gravity oil shows no more 
stain than the gas. Porous sand, containing no stain whatsoever may be 
suspected of carrying water. In examining samples of oil stain, it is 
very convenient to have a fluoroscope, which may detect very light stains 
of high-gravity oil which are not obvious in ordinary light. Best results 
in detection of oil stains are obtained with mercury vapor lamps emit- 
ting light, with wave lengths ranging from 3,300 to 3,800 Angstrom units. 
Lamps giving light of shorter wave lengths, such as the quartz tube, cause 
fluorescence of oil, but also cause much mineral fluorescence in the 
sample, which may confuse the observer. 

Use of Cuttings Descriptions in Study of 
Limestone Reservoirs 

Limestones and dolomites form important oil reservoirs, and cuttings 
descriptions furnish one of the chief sources of information concerning 
them. There are two chief types of porosity in these rocks. The first is 
intergranular porosity which consists of openings between the crystals, 
oolites, or other discrete particles of the rock which in its geometry is 
similar to sandstone porosity. The second is fracture porosity (or fora- 
menular porosity of Bulnes and Fitting ^) which consists of large open- 
ings through otherwise solid masses, such as fractures and vugs. 

The intergranular porosity is easily observed under the binocular 
microscope where the tiny openings may commonly be seen connected 
with each other. The larger openings are not ordinarily visible in their 
entirety under the miscroscope but are indicated by irregular surfaces 
lined by crystals which have been formed in a comparatively large cavity. 
These large openings are much more difficult to detect than the smaller 
pores and ordinarily they have gone unnoticed unless indicated by the 
drilling-time or the manner of drilling. Permeability in the fractures or 
foramenular porosity is almost impossible to determine under the micro- 
scope, since there is no means of knowing how far apart the walls of the 
large openings originally were. 

* Bulnes, A. C, and Fitting, R. U., Jr., An Introductory Discussion of Reservoir Performance of 
Limestone Formations : Petroleum Development and Technology, 1945, Petroleum Division, Trans. Amer. 
Inst. Min. Met. Eng., vol. 160, pp. 181-201. 



Subsurface Logging Methods 359 

Permeability in intergranular porosity can be estimated qualitatively 
by noticing the size of the pores and their apparent interconnection. In 
general, the larger the pores the higher the permeability. The converse, 
however, is not always true. Some dolomites showing very fine porosity 
are shown by core analysis to be surprisingly permeable. However, one 
type of porosity is non-permeable almost without exception. This is 
called pin-point porosity and consists of small isolated holes. Some of 
these holes contain small amounts of asphaltic material and even may 
contain live oil and gas, but commercial production is not developed 
from them. So far, no quantitative results about porosity and permeability 
have been obtained from ordinary cuttings. However, the coarser reverse- 
circulation cuttings have been analyzed for porosity and permeability 
with favorable results. . 

An interesting method of reproducing and visualizing these pore 
spaces is presented by Nuss and Whitney,^ who impregnated limestones 
with plastic and then dissolved the limestone with acid, leaving a model 
of the porosity as a residue. 

Oil and gas stains are ordinarily readily detectable in limestones and 
dolomites. The heavy sour oils, as found in the Upper Permian, leave 
a dark brown stain which is unmistakable. Lighter oils of the Lower 
Permian rocks show good stains, and the very light oils found in the 
Lower Paleozoic strata leave an extremely light stain which is difficult to 
detect under incandescent light, but may be seen in white or ultra-violet 
light. Since most gases carry a small amount of light oil with them, 
they will show slight stains in limestones. Gas-oil contacts commonly 
can be recognized accurately by the darkening of the stain at the top of 
the oil column. Water may be indicated by lightening of the stain and 
black asphaltic residues in the samples. Many dolomites have a char- 
acteristic sheen on crystal faces within the water zone. However, a well 
oil-stained section may produce water upon test. This fact may be at- 
tributed to later movement of the structure which causes shifting of the 
water table. 

In concluding the discussion of cuttings description, it should be 
said that no rigid rules can be given for guidance in describing samples. 
Each geological province and each geological organization have their 
own problems which must be worked out individually. Since the strati- 
graphic sections penetrated by wells are as varied as those on the sur- 
face, there can be no substitute for experience and judgment on the part 
of the geologist. Description of samples should never be allowed to de- 
generate into a mere mechanical process. The better the geological back- 
ground of the person examining samples, the better will be the descrip- 
tion. 



' Nuss, W. F., and Whitney, R. L., Technique for Reproducing Rock Pore Space: An 
troleum Geologists BuH., vol. 31, no. 11, pp. 2044-2049, 1947. 



360 Subsurface Geologic Methods 

Plotting 

After the description of the samples is made, it must be plotted in 
graphic form to make the information easily available for correlation 
and study. It is usual to plot this material on a narrow strip of heavy 
paper or cardboard so that a large number of logs can be laid out to 
compare and correlate the sections. A 3-inch width has been found to 
be most convenient for this purpose. Upon this 3-inch strip the lithologic 
characteristics are plotted on a column ^ to f inch wide with a vertical 
scale of 1 inch equals 100 feet. On this column each 10 feet (1/lOth inch) 
is marked so as to facilitate plotting. The rest of the log strip is used for 
notes on lithologic characteristics that are not easily plotted as symbols. 
It has been found in plotting the lithologic characteristics that strong 
contrasting colors facilitate correlation and comparison of the sections. 
While no standard system of symbols has been adopted, the following 
colors are widely used. 

Light blue = Calcareous limestone 

Dark blue = Dolomite 

Red = Red shale 

Gray = Gray shale 

Yellow = Sandstone 

Green = Salt 

Purple = Anhydrite 

Other colors may be added for local necessities. It has been found very 
helpful also to plot the drilling time next to the lithologic column on the 
log strip. In the case of mechanically taken drilling time, a copy of the 
graph is plotted; with manually taken drilling time, the plotting is done 
as bar graphs — the common scale being 1 inch equals 100 minutes of 
drilling time. 

Porosity and oil stain in the samples are indicated by symbols on 
either side of the lithologic column. These symbols may be either in 
ink or in suitable colors. It is very helpful to make some distinction as 
to probable gas or water stain even though these fluids can not be differ- 
entiated certainly from cutting examination. Sharp changes in lithologic 
character should be indicated plainly on the plotted log. This indication 
usually is done best by having the geologist who examined the samples 
check the plotted log and indicate where he believes the formation boun- 
daries should be, because their exact position is commonly difficult to 
determine from the written description alone. 

All tests and showings should be indicated on the margin of the log 
strip. These should include all drill-stem tests in rotary v/ells, and amount 
and kind of fluid in cable-tool holes. It is also helpful to indicate any 
swabbing tests taken, the acid used, amount of nitroglycerine used and 
section shot, and perforations in the casing. Casing seats should be in- 
dicated on the margin of the log with a notation of the diameter of the 
casing and the amount of cement used in setting the casing. Total depth. 











Andrews County 




Unit/. 


BLK. 
























Unii/ersi-fi/ ^o. 2 














commenced: 1 1 - 10 - 4-2 












completed: 12-19-42 




ELEVATION 


6/4' Fr: S.8, 6 73- Fr. 


3ZZZ Z.&5. 


E. L/nes of J^M/./4- 


pponiirxiow 


F.// 


B 


-O.f- 


'fl 






LEGEND 

1/ / / /I Delomif* 
]/■'.-'■/■':/ A S^nJy Dolomltm 



'- - -^ ■»- I Anhydritt 

■ '—'- ^~-^ :l Sandy Sh^lf 

■^\y:^-iiy--\ Sand 

:-r-:-r-v--i SA*/e 



Oil SAow oJ 
Oil Pay ,J 



. Gr*y Srlnelu Coifiie Slight fo fair porosity fJeoit Stttiri 
Safur*tcel\ SligfiUy poroux 

Dot. Gr»<j Sh«l9 



S/ip dogs in hole 
ti/ar porous Dot- 



Tan Oil SUin^ Slightly anhy. Dot. 



Figure 151. Type of sample log of producing section. Lithology 
is plotted on a percentage basis. Drilling-time data are re- 
corded to right of lithologic column. Note slow penetration 
rate through anhydrite above 4,000 feet. 



362 Subsurface Geologic Methods 

location, and initial production of the well should be shown in the heading 
of the log strip. 

Using Sample Logs 

The first use of these sample logs is stratigraphic correlation by 
laying the logs alongside each other and matching bed for bed as far as 
possible. In doing this the geologist must keep in mind probable lateral 
gradation, probable contamination of samples, and probable changes in 
intervals from one horizon to another. Care should be taken not to 
correlate over any greater distance than is necessary. For correlation 
purposes one tries to select wells close together and extending as far as 
possible along the lithologic strike. This information can be used for 
construction of cross sections ^ and stereograms ^ to show stratigraphy. 

After these correlations are made and the stratigraphic section estab- 
lished, zones can be found whose tops will make good index beds for 
structural mapping. In selecting such horizons one must consider that 
the depths of samples from v/hich the logs were made may not have been 
corrected for the lag in coming to the surface. Therefore, especially in 
zones far below the surface, the geologist must allow for this correction 
or must correct his datum points by the drilling time, if available. 

Another important use of sample logs and descriptions is in the 
analysis and evaluation of any pay zone, especially in limestone reservoirs. 
This may be done by tabulating the pay sections in columns, as shown in 
table 21. The first column shows the depth of each sample in the pay 
section, the second column shows the net feet of pay which is the percent- 
age of porosity and saturated material in the sample times the sample 
interval, ai]^ the next columns give the quality of the pay which is the 
geologist's estimate of the porosity and permeability in a qualitative 
manner, such as slight, fair, medium, and good. The next column shows 
the probable acre-foot recovery from the pay. This figure is derived from 
core analyses in this field, if available, or from sample descriptions of pay 
in fields where the ultimate recovery is reasonably well established from 
past production. The next column in the analysis is the recovery per 
acre, which is the acre-foot recovery times the net feet of pay. This gives 
the recovery per acre for each sample interval. The total of this column 
gives the recovery per acre for the well. Of course, this is a volumetric 
estimate of the recoverable oil from any well, and since limestone reser- 
voirs are commonly heterogeneous in their composition, this recovery 
estimate falls within wide limits of error. Well spacing, of course, has a 
considerable effect on ultimate recovery. Thus, if the acre-foot recovery 
figures are derived from wells differing in spacing from the well under 
analysis, due allowance must be made for this difference in spacing. 

' Hills, J. M., Rhythm of Permian Seas: Am. Assoc. Petroleum Geologists Bull., vol. 25, no. 2, 
pp. 217-255, 1942. 

' Lewis, F. E., Position of San Andres Group, West Texas and New Mexico: ibid., voL 25, no. 1, 
p. 73, footnote 1, 1941. 



Subsurface Logging Methods 



363 



TABLE 21 

Detailed Recovery Estimate, University Well No. 2, Andrews County, Texas 



Depth in 






Net Feet 


iS'et Feet 


Net 


Feet 


Net Feet 


Acre- Foot 


Recover 


Feet 


Remarks 




Slight 


Fair 


Me 


dium 


Good 


R 


ecovery 


Per Acre 








Porosity 


Porosity 


Porosity 


Porosity 






(Barrels 1 


4380-85 


Stained 




1 












75 


75 


90 


Stained 




1 












75 


75 


4400 05 


Sat. V. si. 


por. 


2 












50 


100 


10 


Sat. oolitic 




3 












100 


300 


15 


Sat. isolate 


d por. 


3 












50 


150 


20 


Sat. gran. 
SI. pur. 


V. 


4 












50 


■ 200 


25-30 


Sat. V. si. 


por 


4 












50 


200 


35 


Sat. V. si. 


por 


1 












50 


50 


40 


Sat. V. si. 


por. 


2 












50 


100 


45-50 


Sat. si. por. 


1 












100 


100 


55-60 


Sat. gran. 




2 












125 


250 


65-75 


Sat. 




4 












150 


300 


4500-05 


Stained 




1 












75 


75 


10 


Sat. 




1 












100 


100 


15-20 


Sat. 




5 












75 


375 


25 


Sat. 




5 












75 


375 


30 


Sat. gran. 




5 












100 


500 


35 


Sat. gran. 




3 












100 


300 


40 


V. si. f. p 


or. 


5 












40 


200 


45 


Sat. gran. 




5 












100 


500 


50 


Sat. cryst. 


si. gran. 


3 












50 


150 


55 


Sat. cryst. 




I 


2 










200 


600 


60 


Sal. gran. 




4 












150 


600 


05 


Sat. gran. 




1 
65 


1 

3 










200 

Tota 


200 
5875 



Generally, it is found that this volumetric estimation is somewhat above 
an estimate made from pressure decline or production decline curves, 
and allowances should be made. In spite of its imperfections, this method 
of valuation is the only one available in many fields in which cuttings 
samples have been kept but no other type of reservoir information is 
available. 

A somewhat similar method may be used for sandstone reservoirs. 
However, since sandstone reservoirs are much more uniform in their 
porosity and permeability, it is commonly possible to assign an acre-foot 
recovery for an entire field and to obtain the ultimate recovery by multi- 
plication of the net pay — sand thickness in any well by the acre-foot re- 
covery, rendering detailed pay analysis unnecessary. 

Logs of cuttings samples are also very useful in studying the char- 
acteristics of limestone reservoirs. They give some idea of volume and 
relative permeability of lenticular pay zones. They are a useful supple- 
ment to any coring program undertaken as part of a reservoir study. It 
is often possible to select gas-oil contacts from cutting logs so as to select 
points for plug-back or for packer settings in remedial work. 

Summary 

Samples of well cuttings can be taken from both cable-tool and rotary 
holes in such a manner that the nature of the formations penetrated can 
be determined accurately. These samples can be described by a geologist 
and the description plotted so as to give a graphic section of the formations 
encountered in the well. These sections can be correlated to give a picture 



364 Subsurface Geologic Methods 

of the regional stratigraphy. From the sections, index beds can be selected 
for use in structural contouring. 

From the sample descriptions, volumetric estimates of ultimate well 
yield can be made, reservoir study facilitated, and well remedial work 
guided. 

ELECTRIC LOGGING 

E. F. STRATTON and R. D. FORD 

The electric log consists of a spontaneous potential curve and, gen- 
erally, three resistivity curves. The specific recording practice and the 
type and number of curves vary from one geologic province to another, 
depending upon the nature of the formations and the problems to be 
solved. 

Spontaneous Potential 

The spontaneous-potential (SP) log is used to distinguish between 
permeable and nonpermeable formations, as, for example, sand and shale 
or permeable and nonpermeable limestone. However, a quantitative 
relationship between porosity or permeability and spontaneous potential 
does not exist. Empirical relationships have been found, however, and 
have been established in specific pools for particular formations. 

The spontaneous potential log of a bore hole is a record of the 
potentials measured in the mud along the hole. In fact, the potentials 
are measured between an electrode lowered into the hole and another 
electrode at the surface and are related to an arbitrary constant. As the 
SP log is generally flat in front of shales and shows positive or negative 
anomalies opposite permeable beds, it is convenient to take the line ob- 
tained in front of shales as the base line. 

Spontaneous potential anomalies in a bore hole are due primarily 
to the electromotive forces generated by two different electrical phenom- 
ena. The first of these and the more important is the electrochemical 
cell formed between the drilling fluid, the fluid in the permeable zone, 
and the shale surrounding the permeable section. This may be expressed 
as: 

E = K log^ 

^ Ri (1) 

where £'=electromotive force of spontaneous potential in millivolts. 
i?3=resistivity of drilling fluid in ohmmeters. 
7?i=resistivity of the fluid in the permeable zone in ohmmeters. 
^=factor dependent upon the chemical composition of the two 
fluids and upon the character of the shale adjacent the perme- 
able bed. 
The second of these electromotive forces may result from the filtra- 




Figure 152. Schematic electrical log showing relation- 
ship between electrical characteristics and various 
lithologic types. 



366 Subsurface Geologic Methods 

tion (2) of the drilling fluid into a permeable zone. The principle is a 
recognized phenomenon of electrochemistry (streaming potential) and, if 
effective in a well, may be expressed as: 

E=—^ (2) 

where £=electromotive force or spontaneous potential in millivolts. 
/?2=resistivity of drilling fluid. 
F=pressure differential (atmospheres) between drilling fluid and 

formation. 
F=viscosity of filtering fluid. 

M=complex factor dependent upon the nature of the permeable 
zone, the filtrate, and the filter (mud cake) . 

There may be other factors effective in generating bore-hole potentials, 
but, at present, the phenomena just described appear to be those of major 
importance. 

Whatever their origin may be (electrochemical or electrokinetic) , 
the electromotive forces give rise to a current, which flows through the 
permeable layers, then spreads into the adjacent impervious formations, 
and returns through the mud filling the hole. The SP anomalies cor- 
respond to the drop of potential created by the circulation of the current 
in the hole, and thus measure only a part of the total electromotive forces. 
Consequently, the characteristics of the SP log, and particularly the ampli- 
tude of the anomalies, are a function of several factors, such as the salini- 
ty of the mud and of the formation fluid, the resistivity of the surrounding 
formations, bed thickness, hole diameter, amount of shaly material in the 
permeable bed, and depth of mud invasion. 

Fresh-Water-Bearing Formations 

The SP developed by a fresh-water-bearing formation is usually very 
small (fig. 153), frequently nonexistent (fig. 154), and sometimes reversed 
(fig. 155) , as compared with the SP across a salt-water-bearing formation. 
As most drilling fluids are comparatively fresh and as the electrochemical 
effect has been recognized as being generally preponderant, it is obvious 
from the formula (1) describing the action of the electrochemical cell 
that, when the resistivity of the drilling fluid is appreciably higher than 
that of the formation water, the SP is high and negative; when the two 
are the same, the SP is zero; and when the drilling-fluid resistivity is 
lower than the formation-water resistivity, the SP is positive. Therefore, 
in the case of fresh-water sands and a fresh mud, SP's usually are small. 

Salt-Water-Bearing Formations 

The SP developed by a salt-water-bearing formation is generally 
sharp, has an appreciable magnitude up to 100 or 200 millivolts, and 
is negative with respect to the surrounding shale or nonpermeable forma- 










































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Figure 153. Log shov.ing SP to be very small. Mud resistivity is close to formation- 
water resistivity. 



368 



Subsurface Geologic Methods 



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Figure 155. Log showing polarity of SP reversed. Mud resistivity is less than 
formation-water resistivity. 



370 Subsurface Geologic Methods 

tions (fig. 156). Such a behavior is entirely in agreement with results to 
be expected from a consideration of the formulae as described above when 
the drilling fluid has a higher resistivity (lower salinity) than the forma- 
tion water. The presence of oil in the shaly sand will often lower the SP 
(fig. 157). 

Effect of Porosity and Permeability 

As noted beforehand, there is no quantitative relationship between 
SP and porosity or permeability. However, in a particular section, marked 
variations in the magnitude of the spontaneous potential generally are 
associated with changes in the physical properties of the formation, 
although such changes can be identified only in a qualitative manner. 
For example, in a sand with only 50 millivolts, SP in a section where 
the sands average, say, 100 millivolts, the lower SP may be the resultant 
of a bed-thickness effect, of shaliness, or of an increased resistivity. 
Shaliness probably would mean less permeability, whereas increased re- 
sistivity probably would indicate less porosity or less water saturation. -^^ 

Effect of Drilling Mud 

It has been seen that the SP is directly affected by the resistivity and, 
therefore, by the salinity of the drilling fluid. Considering a salt-water- 
bearing formation, the SP will decrease with decreasing resistivity or in- 
creasing salinity of the drilling mud. Not only is there a decrease in the 
magnitude of the SP, but the anomalies lose definition and the log becomes 
featureless. Figure 158 is a typical comparison of two logs in the same 
well, one with a low-resistivity (salty) mud, the other with a normal mud. 

Mud-resistivity measurements are given on the log headings as well as 
the temperatures at which the measurements were made. Since drilling- 
fluid resistivities vary inversely with temperature and since well-bore tem- 
peratures usually are different from surface temperatures, corrections have 
to be made for mud resistivities measured at the surface in order to 
evaluate their effect in the hole. For example, a mud with a resistivity of 
2.0 ohms at 64° F. will have a resistivity of only 0.70 ohms at 200° F. 

Figure 159 shows the variation of resistivity of a sodium-chloride 
(salt) solution with temperature and with changes in salinity expressed in 
parts per million. 

Factors Influencing Resistivity of Drilling Fluids 

The factors that influence the resistivity of drilling fluids are the fol- 
lowing: 

1. Temperature. Resistivity decreases with increasing temperature 
(fig. 159). 

2. Sodium-chloride salinity. Resistivity decreases with increasing 
sodium-chloride salinity (fig. 159) . 



"Doll, H. G., The S. P. Log: Theoretical Analysis and Principles of Interpretation: Am. Inst. Min. 
Met. Eng. Petroleum Technology, Sept. 1948. 




Figure 156. Negative SP in salt-water-bearing 
formations (sands). Mud resistivity is 
greater than formation-water resistivity. 



372 



Subsurface Geologic Methods 



3. Barite and limestone. Resistivity tends to increase slightly with 
the addition of these common weighting materials.^^ 

4. Cement and sodium bicarbonate. The addition of either or both 
of these materials tends to reduce resistivity. 

5. Sodium pyrophosphate. Resistivity decreases but not uniformly as 
it does with increasing temperature or sodium chloride salinity. 



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Figure 157. Log showing less SP (self potential) across an oil-bearing formation as 
compared to SP of a salt-water bearing formation. Note also decrease in re- 
sistivity opposite water-bearing interval. 



'^ Sherborne, J. E., and Newton, F. M., Factors Influencing Electrical Resistivity of Drilling Fluids: 
Am. Inst. Min. Met. Eng. Tech. Pub. 1466, Mar. 1942. 



Subsurface Logging Methods 



373 



6. Quebracho. The eflfect on resistivity of quantities generally used 
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7. Starch (i.e. "Impermex") . The resistivity is unaffected except as 
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Figure 158. Comparison of two logs in the same well, one with a low resistive (salty) 
mud, the other with a normal mud. 

Effect of Bed Thickness 

Research has indicated that, all other factors remaining the same, 
when the bed is less than four times the hole diameter in thickness, the 
SP will decrease with decreasing bed thickness. 

A section composed of interbedded thin shales and permeable sands 
may have much less SP than the sand would have if the thin shale beds 
were not present. This is an important factor in evaluating thin sand zones 
and explains the good production obtained sometimes from sandy zones 
with low SP.12 

Effect of Special Muds 

The normal constituents of drilling muds, natural clay, "aquagel," 
"baroid," starch, and other additives have no effect on the SP log except 



•Doll, H. C, op. cit. 



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Subsurface Logging Methods 



375 



as they change the resistivity of the mud. On the other hand, two muds 
used occasionally, silicate and oil-base, affect the SP in a manner unre- 
lated to resistivity. 

A positive SP generally is recorded in a silicate mud opposite a 
permeable sand, although the mud resistivity is higher than the resistivity 





















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Figure 160. Example of a log made in sodium-silicate mud showing reversal of 

SP curve. 



of the formation water (fig. 160) . The anomalies are small but sharp 
and are easily seen when the proper SP scale is used. 

SP logs in true oil-base muds, those with a water content of less than 
five percent and a resistivity of several thousand ohms, usually are not 
satisfactory for distinguishing between permeable and nonpermeable for- 
mations or for locating formation boundaries. On the other hand, normal 
SP logs are obtained in oil-emulsion muds. Such muds are a mixture of 



376 Subsurface Geologic Methods 

water-base and oil-base mud, and the resistivity generally has a value of 
from two to ten ohms m"/m. 

Effect of Variation in Hydrostatic Pressure 

It has been demonstrated many times that a change in the hydrostatic 
pressure exerted by the mud column on a permeable formation will change 
the magnitude of the SP. An increased pressure increases the SP while 
a lowered pressure lowers the SP. Since the change generally occurs op- 
posite only the permeable zones, these can sometimes be located and quali- 
tatively compared by measuring the SP at different hydrostatic pressures. 
Such a log is called an "SPD," spontaneous-potential-differential, log. 

Resistivity 

Rock formations, except for example, massive sulphide ore bodies 
and graphitic beds, are capable of transmitting an electric current only by 
means of the absorbed water which they contain. They would be non- 
conductive if they were entirely dry. The absorbed water containing dis^ 
solved salts constitutes an electrolyte able to conduct the current. The more 
electrolyte contained in a formation and the richer this electrolyte in dis- 
solved salts, the greater the conductivity and therefore the less the resis- 
tivity of the formation. Fresh water, for example, has only a small amount 
of dissolved salts and is, therefore, a poor conductor of an electric cur- 
rent; salt water with a large amount of dissolved salt is a good conductor. 

Electric-logging practice is to measure, not the conductivity, but its 
reciprocal, the electrical resistivity. This is the resistance of a volume of 
rock having a unit of length and a unit of cross section. The resistivity of 
rocks is expressed in ohm meter squared per meter (ohms m'-/m) or 
ohmmeters. This has been found to be a convenient unit for practical pur- 
poses, giving values between a fraction of an ohm and several thousand 
ohms. 

The resistivity measured in a drill hole and recorded on the electric 
log is called "apparent resistivity." This will vary from the "true forma- 
tion resistivity" as a function of bed thickness, electrode spacing, diameter 
of the bore hole, resistivity of the drilling mud, and, in the case of perme- 
able formations, the nature of the invaded zone.^^ It is not feasible with 
present measuring procedures to eliminate these effects; however, the in- 
fluence of bed thickness on apparent resistivity is negligible if the forma- 
tion is many times as thick as the AM spacing of the "normal" curve or 
several times as thick as the AO spacing of the '"lateral." The other factors 
noted above can be taken into account and true resistivity determined from 
the apparent resistivity by the use of resistivity-departure curves.^^ 

The electric-log resistivity or apparent resistivity is satisfactory for 
all problems except the determination of the fluid content of permeable 

*' Doll, H. C, Legrand, J. C, and Stratton, E. F., True Resistivity Determination from the Electric 
Log — Its Application to Log Analysis: Am. Petroleum Inst. Drilling and Production Practice, p. 215, 1947. 
^* Resistivity Departure Curves, Schlumberger Well Surveying Corporation, Sept. 1947. 



Subsurface Logging Methods 377 

zones. The true resistivity required for this latter work may be obtained 
from the departure curves or sometimes directly from the long normal or 
lateral curve. 

Experience and research have proved the general over-all utility of 
the multi-electrode method for making resistivity measurements, as it 
minimizes the effect of the drilling fluid and the well bore, and it makes 
possible a direct comparison of the several recorded resistivity curves. 
Multi-electrode recording, as distinguished from single- or point-electrode 
measurements, is made with a system of four electrodes; two of these are 
current emitting and two are for potential measurement. The curves re- 
corded are termed "normal" or "lateral," depending upon the electrode 
arrangement. 

Terminology 

Electrodes — The current electrodes are designated "A" and "B," the 
measuring electrodes "M" and "N." Common practice is to have the two 
current electrodes, A and B, and one potential-measuring electrode, M, 
in the hole with the other potential electrode, N, at the surface. Some of 
the electrodes in the hole are mounted on a mandrel, called a "sonde," 
which serves as a guide and weight for the cable. 

Normal Curve — A normal curve is a resistivity log recorded with the 
four-electrode system where the distance between one current and one 
potential-measuring electrode, AM, is of primary importance. The posi- 
tion of the other current electrode, B, is relatively unimportant as long as 
the distance, AM, is small as compared to AB (fig. 161). 

Amplified Normal Curve — A resistivity log recorded with the normal- 
curve-electrode arrangement but using an amplified or exaggerated scale is 
an amplified normal curve. For example, the normal curve might be re- 
corded on a 20-ohm scale, the amplified normal with a 4-ohm scale. 

Long Normal Curve — A long normal curve is a resistivity log re- 
corded with the same electrode arrangement as the normal but with the 
distance, AM, several times as great as the normal. 

Lateral Curve — A resistivity log recorded with the four-electrode sys- 
tem, where the distance between one potential-measuring electrode and a 
point midway between the two current electrodes, AB, is of primary im- 
portance, is a lateral curve. The distance AB is small as compared with 
the distance AM (fig. 162). 

Long Lateral Curve — A resistivity log recorded with the same elec- 
trode arrangement as the lateral but with the distance between M and the 
midpoint of AB longer than that of the regular lateral is a long lateral 
curve. 

Electrode Spacing 

Normal Curves — The spacing is considered as the distance, AM, be- 
tween the current electrode, A, and the potential-measuring electrode, M. 
Depending upon the geologic province, this spacing varies between eight 



378 



Subsurface Geologic Methods 



inches and eighteen inches for the normal curve and between five and six 
feet for the long normal curve and is indicated on the Schlumberger log 
heading as "AM." 

Lateral Curves — The spacing is considered as the distance between the 
potential-measuring electrode, M, and a point midway between the two 
current electrodes, AB. This spacing is always large as compared with the 
distance, AB, and is indicated on the Schlumberger log heading by "AO" 
(fig. 162). 

Depth of Investigation — -The depth of investigation is an indefinite 
matter since, for both the normal and lateral arrangement, it varies with 
many factors. It can be stated, however, that in general the greater the 
AM or AO spacing the greater the depth of investigation. 

Characteristics of Resistivity Curves 

A resistivity log has primarily a twofold purpose: one, to locate and 
determine the boundaries of all resistive formations; the other, to deter- 
mine the fluid content, both qualitatively and quantitatively, of permeable 



<2) 



AB - Current. Electrodes 
MN - Potential Electrodes 

AM small compared to AB 



a 



(AM on log heading) 



AB - Current Electrodes 
MN - Potential Electrodes 
AB small compared to AM 



T 



(AO on log heading) 



__J_ 



Figure 161. Schematic diagram showing 
arrangement of electrodes for record- 
ing normal resistivity curves. 



Figure 162. Schematic diagram of elec- 
trode arrangement for recording lat- 
eral curves. 



Subsurface Logging Methods 379 

formations. The first condition, with a normal curve, is achieved best by 
a short electrode spacing, AM; the second, by using a longer electrode 
spacing in order to minimize the effects of the drilling fluid, the diameter 
of the hole, and the invaded zone. As a result two or more resistivity 
curves, one with a short spacing and others with a somewhat longer 
spacing, are commonly recorded and presented on each log. 

The particular behavior of normal curves in resistive beds of various 
thicknesses is illustrated in figure 163. It is to be noted especially that a 
resistive bed equal to the electrode spacing gives no indication; a resistive 
bed where thickness is less than the electrode spacing is shown as an in- 
verted anomaly. 

The lateral-type curve, with the spacings commonly employed, is 
usually adequate to minimize the eiOfect of the invaded zone and, at the 
same time, to indicate the position of resistive zones. Figure 164 indicates 
the behavior of the lateral curve with resistive beds of different thicknesses. 
Note that it shows beds of all thicknesses, but that the top boundary of 
formations whose thickness is greater than the AO spacing is indefinite, 
and true values are shielded out for a distance equal to the AO spacing. 
The actual thickness of beds less than AB is exaggerated by an amount 
equal to the distance between the current electrodes. Below thin resistive 
beds an abnormally low resistivity is measured for a distance equal to the 
AO spacing regardless of the nature of the forma-tion opposite the section 
(fig. 165). 

Application of the Electric Log 

Correlation — The utility of the electric log in detailed structural pool 
studies or in general stratigraphic investigations is well known. Figure 166 
is a typical correlation study in the Midcontinent area. 

Distinction between Porous and Permeable Formations and Non- 
porous and Nonpermeable Formations — The spontaneous potential curve 
usually indicates permeable formations containing saline interstitial water 
by a marked negative anomaly. This characteristic is common for sands as 
well as limestones or dolomites and for shallow as well as deep forma- 
tions. Formations containing fresh interstitial water, on the other hand, 
are usually indicated by their lack of SP anomaly or by a positive 
anomaly. 

An electric-log analysis through a limestone or cemented-sandstone 
section, where permeable zones occur interbedded with otherwise nonper- 
meable beds, needs particular mention. The permeable zones, whether oil- 
or water-bearing, are usually more conductive (less resistive) than the sur- 
rounding nonpermeable formations because of the saline interstitial water. 
The resistivity log exhibits, therefore, a lower value across the permeable 
zones than across the nonpermeable ones (fig. 167) . The anomalies on 
the SP log spread above and below the permeable beds to such an extent 
that permeable-zone boundaries are not determined easily by a cursory ex- 



380 



Subsurface Geologic Methods 





















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Figure 163. Log illustrating effect of resistive beds of various thicknesses on normal 

curves. 

amination. The latter problem has been discussed in detail by Doll,^^ and 
reference to his paper will indicate that permeable-zone boundaries in an 
otherwise resistive and nonpermeable formation can be determined with 
accuracy by a careful study of the SP log in conjunction with the resistivity 
diagram. 

The Location and Exact Depth of All Formations — ^A stratigraphic log 
is obtained indicating the presence and depth position of all formations. 
This practically eliminates the possibility of passing up a potentially pro- 
ductive oil- or gas-bearing formation. 

Sand Studies — Changes in the physical characteristics of reservoirs 
can be studied, aiding the solution of many exploration and production 
problems. 

Determination of Thicknesses — ^The thicknesses of all formations can 
be determined, and the net producing thickness of sand reservoirs can be 
calculated. 

Fluid-Content Determinations — It is possible in most cases to dis- 
tinguish between an oil or gas reservoir and a water-bearing formation; in 
many sand reservoirs a quantitative determination of the percentage of 
void space containing oil or gas or, conversely, the percentage of void 
space containing interstitial water can be made. 



" Doll, H. G., op. eit. 



Subsurface Logging Methods 



381 





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Figure 164. Log illustrating behavior of lateral curve with resistive beds of different 
thicknesses. Comments on curves to right of depth column: dashed line repre- 
sents the lateral A0 = 15'; solid line represents the normal AM = 16"; solid 
line curve at extreme right represents the normal AM = 63". 

It has been proved experimentally that the resistivity of a water-satu- 
rated permeable formation is related to the resistivity of the water, to the 
amount of void space, and to the size, shape, and distribution of this space. 
This relationship may be expressed as:^^ 

Ro=FR^ (1) 

where Ro = resistivity of formation 100-percent water-saturated. 
F = formation-resistivity factor. 
Ry, = resistivity of water saturating the formation. 

The factor F has been shown, in general, to be equal to : 



F = 



(2) 



where p = fractional porosity. 

m = proportionality factor. 

The factor m is related to size, shape, and distribution of the void 
space. Experimental data have shown that it varies from 1.3 to somewhat 
over 2.0. The lower values are generally found in unconsolidated sands, 
the higher in consolidated sands. 

A knowledge of the value of Ro, the resistivity of a formation 100- 
percent water-saturated, is fundamental to any interpretation for fluid 



" Archi. 
istics: Am. 



, G. E., The Electrical Resistivity Log as an Aid in Determining Same Reservoir Character- 
Inst. Min. Met. Eng. Tech. Pub. 1422, Jan. 1942. 



382 



Subsurface Geologic Methods 



content, whether it be qualitative or quantitative. Consider a sand con- 
taining water with a salinity of 50,000 p. p.m. at 100° F. and another of 
identical characteristics containing water with a salinity of 5,000 p. p.m. 
at the same temperature. Reference to the chart of figure 159 shows 
that in the first the water resistivity would be about 0.10 ohms m^/m; in 
the second, it would be about 0.85 ohms m^/m, which, applying the for- 
mula (1) , would mean an eight-fold increase in the resistivity of the water 
sand. 

A decrease in porosity causes an increase in resistivity, while an in- 



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curve. 



crease in porosity causes a decrease in resistivity. This factoi is not 
especially important for small changes in porosity — less than five percent 
— but must be given major consideration when there are marked changes 
in percentage porosity. 

The evidence indicates clearly that fluid-content interpretation can be 
made if the resistivity of the sand is known when it is 100-percent water- 
saturated. These data may be obtained in one of two ways, either (1) di- 
rect from the electric log, or (2) by laboratory resistivity measurement 
on a core. 

The second method requires a measurement of the resistivity of a 
core sample 100-percent water-saturated. This information permits a de- 
termination of the formation-resistivity factor; and, thereby, knowing the 
salinity of the water in the formation, one can calculate the resistivity of 
the formation when it is completely water-saturated. 

It has been shown by a number of investigators that the resistivity of 
an oil reservoir is, among other things, related to the percentage of water 




Figure 166. Use of electric log for correlation 
in Midcontinent area. 



384 



Subsurface Geologic Methods 



it contains. The relationship for sands with water saturations higher tlian 
about fifteen percent is approximately 

where S = fractional water saturation. 

Ro = resistivity of sand 100-percent water-saturated. 
R = resistivity of sand partly saturated with oil or gas. 

Since the factors Rg and R can be obtained from the electric log, a pro- 
cedure is available for using the log as a tool in quantitative reservoir 




Figure 167. Common characteristics of an electric log in limestone. 

study. In order to facilitate the use of the relationship expressed above, 
the equation has been placed in graphic form in figure 168. Actual quan- 
titative log interpretations are only as accurate as the data on which they 
are based. The resistivity values for Rg and R must be obtained practi- 
cally unaffected by invasion of the drilling fluid, the salinity of the fluid, 
the hole diameter, or the thickness of the bed. Such resistivities are com- 
monly termed "true resistivities" as contrasted to "apparent resistivities," 
a value related to true resistivity but affected by the factors noted above. 
Experience has indicated that the long normal and lateral curves are gen- 
erally adequate to minimize the effect of drilling-fluid invasion and hole 



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SCHLUMBERGER WELL 
SURVEYING CORPORATION 

Relationship between Resistivity and 
Interstitial Woter Saturation. 

Fundamental Formula: 

*' R 

W = Interstitial Water Saturotion. 

Ro = Resistivity of formation 100% water 
soturated. 

R = Resistivity of oil or gas bearing 
formotion. 
Curves are plotted for constant values of R 






































































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386 



Subsurface Geologic Methods 




Figure 169. Combination log and core analysis and production results. 



Subsurface Logging Methods 387 



Explanation of figure 169 
LOG ANALYSIS 

1) Water saturation S 2) Formation water resistivity Rw 

D 

At water level Ro = 0.3 ohms SP = -70 logio p^ = -90 mv. 

= 12% approx. thus, Rw = 0.05 ohm at BHT 

3) Porosity p 

r Ro 0.3 . 



= Sfh 



30% for m = 1.5 



CORE ANALYSIS PRODUCTION RESULTS 

Average Porosity 30% Perforated: 6581-6594 

Average Permeability 1000 md. i .,. i n j .• n-r lli j oao adi 

Initial Production: 97 bbls. per day 30 API 

Connate water determined by _ , , 

restored state methods 10-12% ^'' °" '^''°^ ^^^^/^ 

Gas increasing with time. 



386 



Subsurface Geologic Methods 




Figure 170. Portion of electrical log, showing typical profile pattern reflecting inter- 
bedded limestone, shale, and a salt-water-bearing sandstone. 



Subsurface Logging Methods 



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Figure 171. Portion of electrical log showing typical profile pattern of a section con- 
sisting of thin interbedded sandstones and shale (Cretaceous of "Wyoming). 





















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Figure 172. Portion of electrical log showing typical profiles produced by an oil sand, 
and a comparison with the core record and core analysis (Mississippian of Illi- 
nois) . 



390 



Subsurface Geologic Methods 




Figure 173. Comparison between electrical log and core record in an alternating 
sandstone-shale section (Texas Tertiary). 











20 




Subsurface Logging Methods 

10 10 20 


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391 
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Figure 174. Portion of electrical log through a section composed eesentially of shale, 
thin limestone, oil sands, and salt-water-bearing sands (Texas Tertiary) . 



392 



Subsurface Geologic Methods 




Figure 175. Comparison of an electrical log and core record in a section composed 
of oil sand, salt-water-bearing sand, and shale (Texas Tertiary) . 



diameter; however, as shown in figure 163, the resistivity is a function of 
bed thickness, which becomes of major importance for thicknesses close to 
that of the electrode spacing. These factors must be taken into considera- 
tion when true resistivities are determined. 

The foregoing discussion shows clearly the potential value of the 
correct use of an electric log. Experiments and research are being carried 
out today that will soon make possible a more complete evaluation of true 
resistivity and therefore an even more thorough formation study. The 
electric log is much more than merely a correlation tool, and with a 
complete understanding of the factors influencing the log much valuable 
information may be obtained. 



Subsurface Logging Methods 393 

INDUCTION LOGGING AND ITS APPLICATION TO LOGGING OF 
WELLS DRILLED WITH OIL-BASE MUD 

H. G. DOLL 

The measurement of the resistivity of the formations traversed by 
drill holes has become standard practice in oil-well drilling during the 
last twenty years. The technique used requires that direct contact be made 
with the mud filling the bore hole by means of electrodes connected to the 
insulated conductors of the supporting cable. A current of constant in- 
tensity is generally made to flow in the surrounding medium through one 
or two of these electrodes called "power electrodes." It produces in the 
surrounding medium, by ohmic effect, potential differences which are pro- 
portional to its average resistivity. These potential differences are picked 
up by one or more measuring electrodes and are recorded continuously 
at the surface of the ground, giving the resistivity log. 

There are cases, however, where a direct contact between the elec- 
trodes and the drilling mud is not possible; for instance, in holes drilled 
with cable tools, which are generally dry, or in holes where nonconductive 
oil-base mud is used in rotary drilling. The conventional electric-logging 
method then requires scratcher electrodes, which are forced by springs 
on the wall of the hole to make direct contact with the formations. In 
some cases the results are fairly satisfactory, but sometimes, particularly 
in wells drilled through hard formations, the measurements are not re- 
liable because of poor contacts with the formations. It is particularly for 
that reason that a new method of electric logging, known as "induction 
logging," has been introduced for resistivity measurements in oil-base 
mud. 

The induction-logging system does not require any direct contact 
with the mud or with the ground. As indicated by the name of the method, 
the formations surrounding the logging apparatus are energized by in- 
duction. To that effect, alternating current of appropriate frequency is 
made to flow through a coil, referred to as the "transmitter," which is sup- 
ported by an insulating mandrel. The alternating magnetic field thus 
created generates eddy currents, which follow circular paths coaxial with 
the hole and the coil system, in the formations surrounding the hole. These 
eddy currents create a secondary magnetic field, which induces an electro- 
motive force in a second coil, referred to as the "receiver," mounted on 
the same nonconductive mandrel at a certain distance called "spacing" 
fr©m the transmitter. 

If the transmitter current is maintained at a constant value, the in- 
tensity of the eddy currents is proportional to the conductivity of the 
ground. Thereby, the conductivity of the ground determines the secondary 
field created by the eddy currents and the signal generated in the receiver. 

As in regulai logging with electrodes, the signal is recorded con- 
tinuously at the surface of the ground while the apparatus is moved along 



394 Subsurface Geologic Methods 

the hole. The record thus produced, which is frequently called an "induc- 
tion log" because of the way in which it is obtained, shows the variations 
of the ground conductivity — and, consequently, of its inverse, the ground 
resistivity — with respect to depth. It is, therefore, equivalent to the 
resistivity log obtained by the conventional method of electric logging 
with electrodes in water-base mud. 

The advantages of the method are more immediate, especially in view 
of the difficulties encountered by the conventional method of electric log- 
ging. This does not mean that the induction-logging method will not work 
in water-base mud; on the contrary, it is believed that in that case also 
the method will have important advantages. Experience in water-base 
mud is, however, still very limited, not only because the available instru- 
ments were all applied to oil-base-mud operations, where they were badly 
needed, but also because certain improvements, which are still being 
studied, are to be introduced for best operation in water-base mud. This is 
why it is felt advisable not to discuss the use of induction logging in water- 
base mud at this time and to wait for the results of field tests that will be 
made for that case. 

In conventional logging practice, the resistivity unit is the ohm-meter. 

Conductivities are expressed in mhos per meter. It is preferred, however, 

to use units of millimhos per meter for induction logging in order to get 

a range of values that does not require an extensive use of decimal figures. 

Accordingly, 

1,000 

C mmhos/m = -^ — — 

K ohm/m 

Thereby, a bed with a resistivity of 100 ohm/m has a conductivity of 
10 mmhos/m. 

Resistivity Measurements by Induction Logging 

The apparatus used for induction logging is shown schematically in 
figure 176; it is in fact a mutual-impedance bridge. It comprises essen- 
tially a transmitter coil T, fed with alternating current by an oscillator, 
and a receiver coil R connected through an amplifier to the recording 
galvanometer. In the absence of any conductive medium around the ap- 
paratus, as, for example, when it is suspended in the air from a wood 
frame high enough above ground, the coupling between the transmitter 
and receiver coils is fully balanced, so that the measuring apparatus reads 
"zero." When the apparatus is in a drill hole, the alternating field set up 
by the transmitter coil produces in the surrounding medium, i.e., in the 
ground, induced currents, generally known as "eddy currents," which 
are proportional to the conductivity of the ground. The electromotive 
force induced in the receiver coil by the eddy currents, referred to here- 
after as the "signal," and designated by E, is proportional to the con- 
ductivity of the ground. If, therefore, the apparatus is properly calibrated, 



Subsurface Logging Methods 



395 



Z= Distance of center "O" of 
solenoid system below 
ground loop 

r= Radius of ground loop 

A= Angle through which the 
two solenoids are seen fromi 
ground loop 




A 



Ground Loop 

Of Unit 

Cross Sectional 

Area 



:^^ 






>An?iplifier a 
'.Oscillator 
Housing 



\ 
\ 

\/ • 

/ 

^ / 
/ 



/ 



Figure 176. Elecetrical principle (A) and apparatus (B) used for induction logging. 
(From Oil and Gas Jour.) 



CONVENTIONAL ELECTRODE LOG 



INDUCTION LOG 




Figure 177. Induction log (right) recorded in oil-base mud, alongside a conventional electric 
log (left) of same well recorded later in water-base mud. 



Subsurface Logging Methods 397 

a measure of the signal constitutes a quantitative determination of the 
conductivity of the ground. 

The signal is amplified and rectified into direct current for trans- 
mission in the cable to the surface where it is automatically recorded. 
A remote-controlled test signal is provided in the apparatus to check the 
calibration. 

The oscillator and the amplifier are contained in a pressure-proof 
housing, called the "electronic cartridge," on top of the coil assembly. The 
subsurface instrument is represented schematically in B of figure 176. An 
induction log recorded in oil-base mud by this equipment is given in figure 
177, alongside the conventional electric log of the same well recorded 
later in water-base mud for comparison. 

When the ground surrounding the coil system is homogeneous, as 
is practically the case for a thick bed which is not appreciably invaded 
by the mud fluid, the conductivity, as measured by the apparatus, is equal 
to the true conductivity of the ground. When, however, the ground around 
the coil system is not homogeneous, as, for example, in the case of a thin 
bed surrounded by formations of appreciably difl'erent conductivity, the 
conductivity, as measured by the apparatus, represents a combination of 
the conductivities of the different media surrounding the coil system and 
is referred to as the "apparent conductivity." This is similar to what 
happens for electric logging with electrodes, where the apparatus also 
measures an apparent resistivity. In both cases, a better approximation of 
the true conductivity can be obtained by applying corrections deduced 
from the departure curves ^^ or correction charts. 

An important advantage of the induction-logging system is that the 
measured values, even without corrections, are already nearer to the true 
values; furthermore, the corrections themselves are much easier to com- 
pute than in the case of logging with electrodes, particularly when in- 
fluence of bed thickness is to be taken into consideration. 

Geometry of Induction Logging 

In the logging method using electrodes for the determination of the 
ground resistivity the flow of current is of the radial type, and it is not 
possible to study separately the influence of the different regions of ground 
surrounding the electrode system. The reason is that the lines of current 
flow cross the boundaries between the different media, such as, for ex- 
ample, the boundary between a given bed and the bed next to it or the 
boundary between the mud and a bed. If the resistivity of any given 
medium is changed, this affects the lines of current flow even in their path 
through the other media. This is the reason that the mathematical com- 

_ ^' Departure curves for electric logging with electrodes have been published earlier in a booklet 
entitled "Resistivity Departure Curves," 1947, by Schlumberger Well Surveying Corporation, Houston, 
Texas. The application of the curves was discussed in a paper on "True Resistivity Determination from 
the Electric Log — Its Application to Log Analysis," by H. G. Doll, J. C. Legrand, and E. F. Stratton, 
presented at the 1947 spring meeting of the Pacific Coast District, Division of Production, American 
Petroleum Institute, at Los Angeles, California. 



398 Subsurface Geologic Methods 

putation of departure curves is rather complicated and can only lead to a 
fair approximation when the beds become thinner and more homogeneous. 
In induction logging the situation is entirely different. If the hole 
is vertical, as will be assumed to simplify the discussion, the lines of 
current flow are horizontal circumferences having their centers on the 
axis of the hole. Since there is generally a symmetry of revolution of 
the ground around the axis of the drill hole, each line of current flow 
remains in the same medium all along its path and never crosses a bound- 
ary between media of different conductivities. On the other hand, if the 
frequency is not extremely high, the reaction of the different circular 
currents on one another can be neglected. In this condition, the action of 
the different regions of ground, which individually have a symmetry of 
revolution around the hole, can be considered separately, and the measured 
signal is simply the sum of the individual signals given by the different 
regions. The consequence is that the theoretical computation of charts or 
of typical logs corresponding to any distribution of ground conductivities 
is always possible, if, of course, there be a symmetry of revolution, as is 
usually the case. 

Conclusion 

The conductivity or the resistivity of formations traversed by a drill 
hole can be determined by the induction-logging method. This new tech- 
nique is particularly useful at present for logging dry holes and holes 
filled with oil-base mud, in which direct contact with the formations is 
difficult to establish. 

The method has great flexibility and is quite promising. Coil systems 
can be designed to give a focusing effect in order to obtain directly on 
the log a more accurate value for the conductivity of beds of finite thick- 
ness. 

Since the different regions of ground generally have a symmetry of 
revolution around the axis of the hole and since, therefore, the induced 
currents have circular paths around that axis, the currents never cross the 
boundary from one region to the other. In these conditions the contribu- 
tions of the different regions to the measured signal can be considered in- 
dependently. For that reason it is relatively easy to compute typical logs 
and correction charts, which should greatly improve the possibilities of 
quantitative interpretation. 



Subsurface Logging Methods 399 

THE MICROLOG 
H. G. DOLL 

In conventional electrical logging, the spontaneous potential (SP) 
log is used to delineate the permeable beds, and the resistivity logs are 
used primarily to provide indications concerning the fluid content of the 
beds. 

When the formations are much more resistive than the mud, as 
happens, for example, in limestone fields, the SP currents are short-cir- 
cuited by the more conductive mud column, with the result that the SP 
log is quite rounded. In that case, the SP log generally gives the ap- 
proximate location of the permeable formations, but it cannot be used 
for an accurate determination of the boundaries of each permeable bed.^^ 

Solutions for the problem of obtaining a better determination of 
the permeable beds in limestone fields were developed from two angles. 
One approach consisted in improvements of the logging of the SP, as 
given by Selective SP logging and Static SP logging. ^^ These new methods, 
which have been described in an earlier paper, give good results when the 
mud is not too salty; but they are still in a somewhat experimental stage, 
mostly because the development efforts have lately been concentrated on 
another approach to the problem, i.e. the microlog. 

The mJcrolog, which is the subject of the present paper, has been 
developed primarily as a means for the accurate determination of the 
permeable beds, where the SP log alone does not give a satisfactory 
answer. For that reason, this new development has found its first field 
of application in limestone areas, where the usefulness of micrologging 
is most obvious. The microlog is, however, also of importance in sand 
and shale formations, if only for a more precise determination of the 
boundaries between successive beds, and for a better evaluation of the 
sand count. 

It is emphasized that the present paper is not intended to give an 
exhaustive and definitive description of the subject. In fact, the application 
of the new method has had a partly experimental character up to the 
present time, and several features of the corresponding technique are 
still being improved. It is possible that some of the improvements now 
under way will modify, to a certain extent, the response of the micrologs 
and the procedure of interpretation. These differences, however, should 
not bring about any fundamental changes in the principle of this method, 
and should rather make its application easier and more reliable. 

Principle of Micrologging 

A microlog is a resistivity log recorded with electrodes spaced at 
short distances from each other in an insulating pad which is pressed 

''Doll, H. G., The SP Log: Theoretical Analysis and Principles of Interpretation: Petroleum Tech- 
nology, vol. II, or Transac'ions AIME. Petroleum Branch, vol. 179, p. 146, Sept. 1948. 

*° Doll, H. G., Selective SP Logging: Paper presented at the AIME Columbus, Ohio Meeting Sept. 
25-28, 1949, and at the San Antonio, Texas meeting, Oct. 5-7, 1949. 



400 Subsurface Geologic Methods 

against the wall of the drill hole. Under those conditions, the system 
measures the average resistivity of the small volume of material — here- 
inafter referred to as a "microvolume" — which is located under the pad, 
and which is, therefore, electrically shielded against the short-circuiting 
action of the mud. Two di£ferent electrode systems, with different depths 
of investigation, are generally used in combination to provide two logs 
that are recorded simultaneously. For both electrode systems, the spac- 
ings are very small — usually one inch or two inches. In the discussion, 
the systems which have the smallest and the largest depth of investigation 
are respectively referred to as the "short spacing" and the "long spacing." 

When the pad is applied to a permeable bed, the mud cake repre- 
sents a substantial porportion of the microvolume. Inasmuch as the mud 
cake has a resistivity Rmc which can be estimated to be only about twice 
the resistivity R^, of the mud, the resistivities recorded through microlog- 
ging — hereinafter referred to as micro-resistivities — are never very high 
opposite permeable beds, and are appreciably related to the resistivity of 
the mud. The other part of the microvolume is constituted by a fraction 
of the solid structure of the permeable bed whose pores are almost com- 
pletely filled by the mud filtrate. The resistivity of that part of the micro- 
volume is, therefore, not much different from the value F X R^ ^^ which 
corresponds to complete mud filtrate saturation, so that it is also directly 
related to the resistivity of the mud. 

It can easily be deduced from these considerations that the microre- 
sistivities measured opposite a permeable bed cannot generally be higher 
than a certain number of times the resistivity of the mud, unless the mud 
cake is very thin, and unless, simultaneously, the formation factor F is 
very high. A corresponding limit Rum can be set at about 20 or 30 times 
the resistivity of the mud for the average case; therefore one of the rules 
of interpretation for the microlog is to classify as most probably imperv- 
ious all formations for which the microresistivities are higher than a 
certain limit Rum directly related to the resistivity of the mud. 

Because of a smaller depth of investigation, the short spacing is more 
influenced by the mud cake, and, therefore, generally gives a smaller ap- 
parent resistivity than the long spacing. This difference between the 
microresistivities recorded using two different depths of investigation is 
called "departure," and is said to be positive when the longer spacing 
gives the larger resistivity. When there is a large percentage of "positive 
departure," the formation can almost certainly be interpreted as perme- 
able. 

When the pad is applied to an impervious bed of low resistivity, 
both spacings measure substantially the same resistivity, which is that of 
the formation, and there is no appreciable departure between the micro- 
resistivity curves. If the resistivity of the impervious bed is very high, 
the microresistivities can differ appreciably from the formation resistivity, 

^^ Archie, G. E., The Electrical Resistivity Log as an Aid in Determining Some Reservoir Character- 
istics: Petroleum Technology, vol. J, 1942. F is the formation factor. 



Subsurface Logging Methods 



401 



but they are both higher than the limit Rum above which beds should be 
classified as impervious. For intermediate values of the resistivity, and 
because of the limited dimensions of the pad, a departure between the 

^ __ __ insulated Cable — ^ 



g \iSt 



M2 

Mi^^Electrodes 



■Steel Springs "^ — 




V/ 



Figure 178. Micrologging apparatus. 

microresistivity curves is sometimes observed on impervious beds; but, 
in that case, the departure is negative; i.e. the longer spacing gives the 
smaller value of the apparent resistivity, so that there can be no con- 
fusion in the interpretation. 

These different features of the interpretation will be discussed in 
more detail in a later section. 



402 



Subsurface Geologic Methods 



Description of the Equipment 

The micrologging apparatus consists essentially of a rubber pad, 
which is pressed against the wall of the drill hole, and in the face of 
which are inserted a certain number of electrodes. Several distributions 
of electrodes have been experimented with. One of the distributions is rep- 
resented in figure 178. The electrodes are nearly flush with the rubber 
surface, or slightly recessed, and each of them is connected by an insu- 



^TL^ 




\\\ h 

Figure 179. Microlog electrical setup. 

lated wire to one of the conductors of the cable used to lower the appara- 
tus into the hole. 

The rubber pad is molded on one of the branches of a spring guide 
whose design is such that the pressure applied to the pad is approximately 
independent of the diameter of the hole, provided that this diameter re- 
mains between certain limits which, for one of the guides presently in 
use, are respectively 4^ inches and 16 inches. The rubber pad fits the wall 
of the drill hole over a substantial area surrounding the electrodes be- 
cause of its shape and the pressure exerted upon it. The pad also shields 
the electrodes from the mud column, while the electrodes themselves are in 
direct electrical contact either with the formation, or with the mud cake 
between the pad and the formation. 



Subsurface Logging Methods 403 

The resistivity of the small volume of ground surrounding the 
electrodes can be measured, for example, by sending a current of known 
intensity / through electrode A (fig. 179), and by measuring with galvan- 
ometer Gi, the potential difference created by that current between elec- 
trode Ml and the reference electrode A^. 

Similarly, a slightly larger volume of ground can be taken into 
account by measuring, with galvanometer G^, the potential produced at 
electrode M3 by the same current. Because the spacing AM2 is twice as long 
as the spacing AMi, the corresponding investigation is also twice as deep. 
This is an important feature of the system; it is possible not only to meas- 
ure the average resistivity of the ground under the pad, but also to deter- 
mine whether or not the resistivity varies with depth from the pad. This 
determination is of significance, for a resistivity variation with depth 
from the pad usually occurs when there is a mud cake. 

Electrode combinations other than AMi and AM2 can, of course, be 
used. When a current is sent through electrode A, it is also possible to 
measure the potential difference between electrodes Mj and M^. The three- 
electrode system AM1M2 is more influenced by the mud cake than the 
two-electrode system AMi, so that a combination of the device AM1M2 
and the device AM2 would generally give a larger departure opposite 
permeable beds than a combination of the two devices AMi and AMo. 

To simplify the wording, the two electrode and three electrode sys- 
tems are called "normal" and "lateral" devices, respectively. It should be 
pointed out, however, that micrologging devices, because they have much 
smaller depth of investigation and because the electrodes are shielded 
from the mud column, are different from the normal and lateral devices 
used in conventional logging.^^ 

On most of the pads presently in use, the three electrodes are placed 
on a vertical line, in the middle of the pad, with a spacing of one inch 
between the successive electrodes. Three electrode combinations which 
are thus obtained are listed below for reference. 

Measuring device 

AMx normal 

AM^ normal 

AM1M2 lateral 
Reference 

V normal (or 1'') 

2" normal (or 2") 

l"xr lateral (or r'xl'') 
The combination of the V x V lateral and the 2" normal is preferred 
at the present time. 

The electric circuit used for the recording of the microresistivity 
curves is somewhat similar to that used for conventional logging. Changes 

^ Doll, H. G., Legrand, J. C, Stratton, E. F., True Resistivity Determination from the Electric Log — 
Its Application to Log Anaylsis: Oil and Gas Jour., vol. 46, no. 20, Sept. 20, p. 297 ff», 1947. 



404 



Subsurface Geologic Methods 



have been introduced, however, to compensate for the high resistance to 
ground of the small electrodes. 

Interpretation of Micrologs 

A discussion of the interpretation of the micrologs will be aided by 
placing the different cases usually encountered in practice into categories; 



R^ (off scale) 



Apparent 
Resistivities 



MicroLog 



75 R, 



I Rj^ Specific resistivity" of the 
ground at distance "x" 

ii 



Rj( Very high (off scale) 
except in mud film 



50 Rr 



25R, 



Rfii 



I, R^ Mud film 



2 f 1x1 

Figure 180. Impervious formation of high resistivity with a mud film of 1/64-inch 

thickness. 



Apparent 
Resistivities 



MicroLog 



2° I" l"xl" 





Rx 


Specific resistivity ot the 
ground at distance "x" 






Rf 


impervious bed R^ = 5Rj^ 




Rm- 






X 



6" 



12" 



Distances from pod 
Figure 181. Impervious bed of low-resistivity shale (category I3), 



Subsurface Logging Methods 



405 



which will be illustrated by schematic drawings (figs. 180, 181, 182, 183) . 
Each category will be given a reference letter, namely, Ix, h, h (Imperv- 
ious beds), Pj, Pg (Permeable beds). These letters will be placed on the 
field examples exhibited (figs. 184 through 189) at the levels of each sec- 
tion of bore hole which can be considered as belonging to the correspond- 
ing category. 



i 



Apparent 
Resistivities 



MicroLog 



R^ Specific resistivity ot the 
ground at distance "x° 



~ R j~ Uncontominated zone 



ini 




Resistivity through 
invaded zone 



Mud cake 



I xl 



6 1 2" 

Distances from pad 



Figure 182. Permeable bed (oil-bearing) invaded by mud filtrate (category P2). 

Rt < Rx„ 



Highly Resistive Impervious Formations (Category Ii) 

In impervious formations of high resistivity — i.e. whose resistivity is, 
for example, more than 50 times that of the mud — all the microresistivities 
are high. Because the wall of the drilled hole is generally somewhat 
rugose, the rubber pad cannot fit perfectly against it, and a sort of mud 
film may remain between the two. For that reason, and al^o because of 
the limited dimension of the rubber pad, the apparent resistivity recorded 
on the micrologs can be substantially lower than the true resistivity for 
that type of formation. Wlaen the formation resistivity is very high, the 
rugosity of the wall, which causes a mud film to remain under the pad, 



406 



Subsurface Geologic Methods 



Rt 



SZL 



Apporent 
Resistivities 

MicroLog 

2" (• l"xl" 

im 



Rjj - specific resistivity 

of the ground at distance x 



Resistivity of 
invaded zone 




^ 



Rf - Uncontaminated zone 



6" 12" X 

Distances from pad 

Figure 183a. Permeable bed (salt-water-bearing) with moderate invasion by mud 
filtrate (category Pa). Rt < Rr^ 



Rf 



Apparent 
Resistivities 

MicroLog 
2" I" fxf 

IBM 



R^ — specific resistivity 

of the ground at distance x 




invaded zone 



JRnfx^. !^R| — Uncontaminated zone 
mud cofer — 



6" 12" ^ 

Olstonces from pad 

Figure 183b. Permeable bed (salt-water-bearing) with little invasion by mud filtrate 

(category Pi). Rt < Ri 



E3_ 



Apparent 

Resistivities 

MicroLog 

2" I" l"xl" 



lU 



R^ - specific resistivity 

of the ground at distance x 

f^Xn invaded zone 




*.j-L-__'v L/Rf " Uncontaminated zone 
mud cakg' r , ■ 



6" 12" X 

Distances from pad 

Figure 183c. Permeable bed (salt-water-bearing) with very little invasion by mud 
filtrate (category Pi). Rt < Ri„ 



Subsurface Logging Methods 



407 



almost entirely controls the microresistivities. Too much importance 
should, therefore, not be attributed to the absolute values determined for 
the microresistivities, nor to the amount and type of departure between 
the two curves. 

The diagnostic is based in this category on the fact that all micro- 
resistivities are superior to a certain limit Rum which, as said before, can 
be taken equal to about 20 or 30 times the resistivity of the mud in the 
average cases. Such high microresistivities on both micrologs could not 
normally be recorded for a permeable formation. Both the invaded zone 
of the permeable formation and the mud cake would contribute to lower 



Rni'0.9 of 85* 



Resistivity 

16' NORMAL 



Microlog Permeability In md 

r NORMAL 




Figure 184. Example of microlog. 



Ratlttivlty 



MlcroL*g 




Figure 185. Example of microlog. 



Subsurface Logging Methods 



409 



the apparent resistivity on the micrologs, and keep at least one of them, 
the 1 X 1-in. lateral or the 1-in. normal, under the limit Rum- 

The case of a compact bed of high resistivity is illustrated in figure 
180, where it is supposed that the formation is more than 200 times as re- 



section Gauge 



Microlog 



Nomial 10 




Bm • I ot 72 F 



Figure 186. Example of microlog. 



Rm • I at 72* F 
)f, > 9* 



«„• 16 of 65* 



S.B 



Resistivity 



Microlog 



Short Normal !'< l' Lotarol 
J^ ISJ. J. 50 100 150 go o 9 S 10 15 20 

2" Normol 
Q Umastont Dovico iqo Q 

4_ - 




Figure 187. Example of microlog in vuggular limestone (EUenburger). 



Subsurface Logging Methods 



411 



sistive as the mud, and that there is, between the pad and the formation, a 
uniform mud film 1/64 inch thick. The order of magnitude of the different 
microresistivities is about the same, but there can be a slight departure 
either positive or negative, depending on the shape of the pad, the rugosit) 
of the wall, the ratio of formation resistivity to mud resistivity, etc. 

Impervious Beds of Low Resistivity (Categories I2 and I3) 

Figure 181 represents an impervious bed whose resistivity has been 



R_=I8 at 80' 



MicroLog 

I'xl" LATERAL 




Figure 188. Example of microlog in shaly sandstone. 

assumed to be five times the resistivity Rm, of the mud. The resistivity 
diagram on the right of the figure illustrates the fact that the resistivity 
is uniform and does not vary with the distance from the pad, as would 
occur for a permeable bed. This diagram also shows the resistivity scale 
in relation to the resistivity Rm of the mud. 

The rectangles at the left of figure 181 represent, at the same scale, the 



412 



Subsurface Geologic Methods 



respective values of the different microresistivities that would be obtained 
in that case and the formation resistivity Rf. The apparent resistivities 
measured on the different microresistivity logs are slightly lower than the 
true resistivity Rt of the formation because of the limited dimensions of 
the pad. 

When the resistivity of the impervious bed is not much different 



2.4 ot 86»F 



MicroLog 

3 4"x34" LATERAL 
5 10 




Figure 189. Example of microlog in shaly sandstone. 

from that of the mud, there is practically no departure between the micro- 
resistivity curves (Category I3), When, on the contrary, the resistivity 
of the impervious bed is appreciably higher than that of the mud, there 
is generally, at least with the pads presently used, a substantial negative 
departure which is characteristic of an impervious bed (Category I2) . 



Subsurface Logging Methods 413 

Permeable beds (Categories Pi and P2) 

At the level of a permeable formation, the rubber pad slides over 
the mud cake against which it is applied, and the mud cake itself is separ- 
ated from the uncontaminated part of the formation by the invaded zone, 
wherein the original fluid has progressively been replaced by mud filtrate. 

Two cases must here be considered, depending on whether the invaded 
zone is less or more resistive than the uncontaminated zone. 

The first case, which is the simplest as far as interpretation of the 
micrologs is concerned, is represented in figure 182, which is similar to 
figure 181 already discussed, except that now the resistivity varies in the 
formation with the distance from the pad. Here again, the abscissae on 
the diagram represent the depths from the pad, while the ordinates show 
the resistivity of the ground at the corresponding depths. 

The resistivity Rmc of the mud cake has been assumed to be twice 
the resistivity R^ of the mud. The resistivity Rx^ immediately behind the 
mud cake, where the permeable bed should be practically saturated by 
mud filtrate, has been taken equal to 10 times Rm', this would correspond 
to a value of 10 for the formation factor, if the saturation by mud filtrate 
is complete, and if the pores are reasonably free of conductive solids. ^^ 

On the left of figure 182 are represented, at the same scale, the ap- 
proximate values of the microresistivities that would be obtained in that 
particular case from the microlog. As can be seen, the departure is positive 
and quite substantial. This result is general when Rt is larger than R^g. 
A large positive departure between microcurves is characteristic of a 
permeable bed, provided, however, that the resistivity measured by the 
Ixl-in. lateral, or by the 1-in. normal, be lower than approximately 30 
times that of the mud (Category P2) . 

The interpretation is less definite for beds, such as salt-water-bearing 
beds, where the resistivity of the invaded zone is larger than that of the 
uncontaminated zone. This is particularly true when the mud is of the 
low water-loss type, with the consequence that the mud cake is thin and 
the formation is invaded by the mud filtrate to only a short distance from 
the wall. 

When the mud cake has an appreciable thickness, and when, simul- 
taneously, the depth of invasion is large enough, the microresistivities 
measured with the different electrode combinations show good positive 
departures (Category P2). This effect is represented on figure 183a. For 
a smaller penetration of the mud filtrate into the permeable bed, the 
departure would disappear (Category Pi), as represented in figure 183b. 
For still less invasion, the departure might even be slightly negative, as 
represented in figure 183c, but this negative departure never exceeds 20 
percent (Category Pi) . 

On the left-hand side of figures 183a, b, c are represented the ap- 

" Panode, H. W., and Wyllie, M. R. J., The Presence of Conductive Solid in Reservoir Rocks as a 
Factor in Electric Log Interpretation: AIEE Meeting, San Antonio, Oct. 5-7, 194£>. 



414 Subsurface Geologic Methods 

proximate resistivities that would be measured on the microlog in the 
different cases, and for the different electrodes combinations with the 
assumption that Rto is about five times as high as the resistivity of the 
mud cake, which again corresponds to a formation factor of about 10. 
With them are also represented the formation resistivities. When the de- 
partures between the microresistivities are small, nil, or slightly reversed 
(within 20 percent), the interpretation can be aided by the fact that these 
microresistivities are larger than the formation resistivity, contrary to 
what would normally happen for an impervious bed of low resistivity, as 
discussed in connection with figure 181 above. It is simpler, however, to 
refer to the SP log to resolve the ambiguity in this case. 

In all the cases represented in figures 180, 181, 182, and 183, the 
microresistivity corresponding to the Ixl-in. lateral could be computed 
with reasonable accuracy. The microresistivities corresponding to the 1 
and 2-in. normals are only approximate, because the effect of the limited 
side of the pad cannot be accounted for accurately with these devices. 

It is interesting to notice that the microresistivity for the Ixl-in. 
lateral is the same for the different cases represented on figures 180, 181, 
182, and 183, which cases differ only by the value of true resistivity Rt 
and the depth of invasion by the mud filtrate. This illustrates the fact that 
the microresistivities, when measured with an electrode combination having 
a very small depth of investigation, are essentially responsive to the 
resistivity Rxg of the invaded zone and to the thickness and resistivity of 
the mud cake. 

Summarized Rules of Interpretation 

From the above discussion, it is possible to derive a certain number 
of simple rules of interpretation for the microresistivity logs of forma- 
tions, whereby two electrode combinations of different depths of investi- 
gation have been run simultaneously. These rules, which will apply in 
the great majority of cases, are schematically represented on chart 1. 

Case I — The two microresistivities are higher than Rumt, that is, 
higher than about 20 or 30 times the mud resistivity. The formation then 
under study is a compact one, and should be interpreted as impervious, 
regardless of the departure (Category li). 

Case II — ^The microresistivity determined by the shorter spacing, 
generally Ixl-in. lateral, is smaller than the limit Rum- ^^ that case, the 
sign and magnitude of departure should be examined, and, in case of 
doubt, the ambiguity resolved by reference to the SP log. This gives the 
following interpretation categories: 

(a) Large negative departure (more than 20 percent) — the forma- 
tion is impervious (Category I2) . 

(b) The departure is not definite enough (less than 20 percent) — 
the microlog alone cannot determine, in general, whether the formation 
is permeable or not in this case, except that, when the resistivities of all 



1 — 1 

>« 
o 

CJ> 

a> 

o 
O 


1— 1 


to 


Or 


Cf 


O 

"> 

<D 

O. 

e 


M 

3 
O 

"> 

<D 
O. 

E 


w 

3 
O 

'> 
w 
Q> 
O. 

£ 


© 

o 
© 

E 

© 
a. 


© 

o 

© 

E 

© 
Q. 


1 


i 


S P trend 
positive 


S P trend 
negative 


i 


R2"< R,"^," 

large negative 
departure 


e ^^^ 
n O O 

i 1 I 
?l 1 s 

© _ 
a: ^ 


Rg" >R,%," 

large positive 

departure 


e 

K 


e 
oE" 

V 

3 

X 
S 

a: 



1 1 



&« 



bC bO 



S ^ 



I/)CA1 



o © 



>|-B 



^ 


M U 


.■^ 


nl 


o 


o o 


«) 


bU 




<+i vw 







e 


S f 


P. 


C 


ftjascc; 




moo 


nj 


etf 


r— 1 


3S 


is 






l-H 


CSJTS< 


bC 


bfl 




41I 








'i 

J3 


eueu 



H^ i4_^ VM 






o 0^ 

A a 
gg 

en CO 



416 Subsurface Geologic Methods 

formations are known to be much higher than that of the mud, the fact 
that the microresistivities are comparatively low gives a good probability 
that this is due to mud infiltration in a permeable bed. The ambiguity, if 
any, can be resolved by noting the trend of the SP curve. If that trend 
is positive, i.e. if the convexity is toward the positive side, or if the SP 
is on the positive limit (shale line), then the bed must be interpreted as 
impervious and the microlog gives its exact boundaries (Category I3). If, 
on the contrary, the trend of the SP log is negative, the bed is permeable, 
and again the microlog gives the accurate boundaries (Category Pj). 

(c) Large positive departure (more than 20 percent) — the forma- 
tion must be permeable (Category Po). 

Remarks 

(1) It is indeed wise to check the trend of the SP log in any case; 
unless this log is completely flat, it should always be possible to determine 
the trend of the SP. 

When the mud is saturated with salt, the SP curve is completely 
flat. In that case, however, there is generally little doubt about the in- 
terpretation of the microlog, because the permeable beds, which are in- 
vaded by the saturated mud filtrate, are the only ones to give microresis- 
tivities lower than the limit Rum, which itself is lower than the true 
resistivity of all impervious beds. The interpretation can then be based 
on the observation of the lows in the microlog, while a good positive 
departure, if it exists, brings a useful confirmation. 

(2) The interpretation is also facilitated by the consideration of the 
conventional resistivity log, insofar as this log makes it possible to evalu- 
ate the true formation resistivity Rf. 

When the value of the microresistivity Rmicro is less than Rum, it 
can reasonably be assumed that a bed cannot be impervious unless Rt is 
comprised between values respectively equal to about Rmicro and 2 Rmicro- 
When Rt is between the two values, the SP log will generally give a 
definite anomaly that will make the interpretation safe. When Rt is not 
between the above limits, there is a strong probability that the bed is 
permeable. 

(3) In the particular case of a highly resistive permeable bed, 
with a comparatively conductive invaded zone, and interbedded with 
shales and highly resistive compact formations, the SP log may show 
no appreciable deflection with respect to the shale line. But, in that case,, 
a good correct departure should be obtained from the microlog. 

Exceptions to the rules 

There are few cases which do not fall within the simple set of rules 
discussed in the previous section. Fortunately, these cases are rare, and 
the ambiguity can generally be resolved if the remarks made in the prev- 
ious section are taken into account. 



Subsurface Logging Methods 417 

A first exception, which is obvious, corresponds to cavings whose 
diameters are larger than the maximum expansion of the spring system. 
If, in such a case, the pad remains a few inches from the wall, the two 
microresistivities will be equal to the resistivity R,n of the mud, irrespec- 
tive of v/hat the formation is. If, on the contrary, the pad happens to be 
near to the wall, the Ixl-in. lateral will measure approximately the resis- 
tivity of the mud, whereas the 2-in. normal will already be substantially 
affected by the formation resistivity. Inasmuch as the formation resis- 
tivity is usually higher than that of the mud, even for a conductive shale, 
there will be a positive departure between the micrologs which could be 
falsely interpreted as indicating a permeable bed. These two cases can 
generally be detected by observing the abnormally low value given by 
the short spacing (Ixl-in. lateral) for which the microresistivity is fre- 
quently close to Rm in that case, and they would not fail to be recognized 
if a section-gauge log were run, as is highly recommended when large 
cavings might exist. In those cases, the microlog indications should be 
disregarded, and the interpretation based on the conventional log and, 
in particular, on the SP log, as far as the determination of the permeable 
beds is concerned. 

The interpretation rules could also be at fault if the mud cake, in- 
stead of being built on the wall, and within the hole, were built within 
the pores of the permeable bed. This case has not been encountered yet, 
but it seems that it could occur in coarse-grain sands. If that did happen, 
the part of the permeable bed where the pores are filled with the mud 
deposit would be a little more resistive than the invaded zone behind, with 
the result that the Ixl-in. lateral would measure an apparent resistivity 
slightly higher than that measured by the 2-in. normal. This would result 
in a negative departure, and the bed might, therefore, on the basis of the 
microlog, be falsely classified as impervious. Here again, the SP log 
would generally settle the question. 

Other exceptions, which have not yet been observed, may exist and 
will reveal themselves when more examples are acquired. For that reason, 
it will always be useful to confront the indications given by the microlog 
with those obtained from the SP log, and also from the conventional 
resistivity logs, supplemented by the section-gauge log when large cavings 
are to be expected. 

Field Examples 

Figure 184 shows an example of a microlog recorded in a sequence of 
shales and limestone. In this instance, most of the compact beds give 
rise to microresistivities which are definitely higher than 30 7?,„, so that the 
discrimination between permeable and impermeable formations is par- 
ticularly easy. The permeability record, provided by core analysis, was 
available in this hole and is reproduced in the figure as a check on the 
indications of the electrical logs. 



418 Subsurface Geologic Methods 

In figures 185 and 186, sequences of sands, sandstones, limestones, 
and shales are exhibited. At the upper part of figure 186, a sharp depres- 
sion in the microlog brings the microresistivity down to approximately the 
resistivity R^ of the mud, a fact which is almost a certain indication of 
caving. This interpretation is confirmed by the section-gauge log. 

Figure 187 shows the behavior of the microlog in the case of a thick 
limestone formation (Ellenburger) , composed of compact zones and of 
fissured zones with vugular porosity. 

Figures 188 and 189 illustrate the features of the microlog in se- 
quences of non-consolidated sands and shales, such as those commonly en- 
countered in the Gulf Coast or in analogous geological provinces. In these 
regions, the conventional SP and resistivity logs differentiate very well the 
different sections where sands or shales are respectively predominant, but 
they are not capable of delineating each separate sand or shale streak, if 
these are thin. A very detailed record of the individual permeable and im- 
pervious beds is obtained from the microlog, as shown on the figures. This 
result is of interest for an accurate determination of the proportion of 
shale and sand in shaly sands, or, in colloquial terms, for the evaluation 
of the sand count. 

Possibilities of Quantitative Interpretation 

To date, the microlog has been used primarily for a qualitative 
determination of the permeable beds and an accurate determination of 
their boundaries. It must be kept in mind, however, that the microresis- 
tivities measured for permeable beds are largely dependent on the resis- 
tivity and thickness of the mud cake, and on the resistivity Rxf, of the 
formation immediately behind the mud cake. It is possible that the de- 
velopments presently under Avay could bring the micrologging equipment 
to a point where R^o and the thickness of the mud cake could be deter- 
mined quantitatively. 

When the first few inches of the permeable bed immediately behind 
the mud cake are practically saturated with mud filtrate, and when the 
mud cake is entirely built outside of the formation, and not partly within 
its pores, R=ro is equal to F Rmf, and, therefore, gives a direct measure of 
the formation factor F if the resistivity R„if of the mud filtrate at the 
corresponding temperature is known. The possibility of determining the 
formation factor in situ and continuously would obviously be of great 
interest because of the close relationship of that factor with the porosity, at 
least when the permeable material is reasonably free from conductive 
solids such as clay. 

It is more difficult to predict whether useful information could be 
derived from the thickness of the mud cake. Since laboratory experi- 
ments -^ have shown that, for a given mud, the thickness of the cake and 



-2 Byck. H. T., The Effect of Formation Pemeability on the Plastering Behavior of Mud Fluids: API 
Drilling and Production Practice, p. 40, 1940. 



Subsurface Logging Methods 419 

the water loss are constants which do not depend on the nature of the 
permeable formation, and particularly on its permeability, it is logical 
to expect that all the mud cakes in a given hole will be found to have 
the same thickness. It is, however, possible that, even if the thickness of 
all mud cakes should be the same in a given well, a determination of that 
quantity would give valuable information about the behavior of the mud 
itself in the drill-hole conditions. If further experience happens to show 
that the mud cake thickness varies from bed to bed, these variations 
could likely be related to some particular properties of the permeable 
formations, which might be of interest. 

Conclusion 

A new electrical logging method, called micrologging, has been 
discussed. This method makes use of electrodes applied to the wall of 
the drill hole under a nonconductive pad which shields them from the 
mud column. Two different microresistivity logs are recorded simultan- 
eously, with two different electrode systems, both of which correspond 
to very small electrode spacings. 

Permeable beds are, in general, clearly indicated on the micrologs 
by a positive departure between the two microresistivity curves. Even 
when the departure is not definite, the interpretation is easy, thanks to 
simple rules based on the magnitude of the microresistivities and on the 
behavior of the SP log. In all cases, the boundaries of the permeable 
beds are determined with great accuracy. 

The microlog is, therefore, an important addition to the conventional 
electrical log, and should contribute to a better and more accurate deter- 
mination of the permeable beds, particularly in limestone territories. 
Because of its accuracy in the determination of boundaries, it is, however, 
probable that the microlog will also render important services in sand 
and shale formations, where it could appreciably increase the accuracy 
of the sand count. 

Acknowledgments 

The writer is indebted to the many engineers of the Schlumberger 
Well Surveying Corporation who, at headquarters and in the field, cooper- 
ated in the development of the method described in this paper. Acknowl- 
edgments are also due to the oil companies for their courtesy in making 
examples available. 

RADIOACTIVITY WELL LOGGING 
V. J. MERCIER 
Radioactivity well logging as practiced commercially in the United 
States and South America is at the present time composed of two curves, 
the gamma-ray curve and the neutron curve. 

The gamma-ray curve is a relative measurement of the natural radio- 
activity occurring in the strata of the earth. Minute quantities of radio- 



420 



Subsurface Geologic Methods 



active materials in one form or another are universally distributed. 
Measurable quantities are found in all kinds of igneous, metamorphic, 
and sedimentary rocks. From laboratory measurements and the experi- 
ence gained in logging more than 15,000 wells, certain conclusions can 
be drawn concerning the relative intensity of radioactivity in different 
kinds of sedimentary rocks. Figure 190 is a chart that demonstrates the 
relative radioactivity values of various formations encountered in well 
logging. Anhydrite, salt, and coal are very low in radioactivity, while 





RADIOAaiVITY INCREASES 


ANHYDRITE 




D 


SALT 




D 


SAND 




II 


LIME 

SHALY SAND 
SHALY LIME 
SANDY SHALE 
LIMEY SHALE 

SHALE 

BENTONITE, ASH, 
ORGANIC SHALE 




1 1 


1 1 




1 1 




1 1 




1 1 


1 RED 1 GREY iBROWNl BLACK j 




1 1 






RELATIVE RADIOACTIVITY RANGE OF ROCKS 



Figure 190. Drawing indicating radioactive values of various formations encountered 

in well logging. 



shale, bentonite, ash, and organic shale have the highest values of radio- 
activity encountered. Producing formations such as sand, limestone, and 
dolomite are relatively low in radioactivity. 

The neutron curve might well be referred to as a "fluid content or 
hydrogen curve," since hydrogen is the controlling factor on the action 
behavior of the curve in well logging. Neutron well logging is the 
process of bombarding the strata with a strong source of fast-moving 
neutrons and recording the secondary gamma rays that have been excited 
by the neutron bombardment. Where hydrogen is present in the strata, 
the neutrons are slowed down or stopped, and this in turn gives a low 
value on the curve. Where there is no, or very little, hydrogen present, 
the response is quite high in value and is shown as a throw to the right. 



Subsurface Logging Methods 



421 



Instrumentation 
The object of radioactivity well logging is to measure the radiations 
emitted by radioactive substances in the rock formations adjacent to the 
walls of the drill hole and to plot the intensity of the radiations in the 



INSTRUMENT TRUCK^ 

AMPUFIERS *UIOM»TIC RECORDtK 




- 
6 6 

® © 
o 


@ 


^A^ 
^ ^ 






Figure 191. Radioactive logging apparatus. (Lane-Wells.) 

form of a graph versus depth. The order of magnitude of the radio- 
activity to be expected in everyday practice is exceedingly small. Average 
midcontinent sedimentary rocks contain from two to twenty micromicro- 
grams of radium per gram of rock. This concentration of radium is so 



422 Subsurface Geologic Methods 

small that one thousand metric tons would be required to extract a few 
milligrams of the radium element. The well-logging instrument employs 
an ionization chamber to observe the radiations. The thick wall of the 
ionization chamber serves two purposes, which are (1) to resist the 
pressure of the fluid head in the well, and (2) to select the most pene- 
trating part of the radiation in the bore hole. The ionization chamber 
contains an inert gas under pressure, in which are immersed two insulated 
electrodes. One of these insulated electrodes is connected to a battery, 
which keeps it at a positive potential with respect to ground. Gamma 
rays passing through this inert gas partly ionize the gas, permitting a 
current flow between the two electrodes. This current is amplified by an 
amplifier in the subsurface detector and is transmitted through a conduc- 
tor cable to the surface, where it is further amplified by surface equip- 
ment and recorded on a pen-and-ink-type recorder. 

The neutron curve is recorded in a similar manner with the excep- 
tion that the strata are bombarded by a very strong source of neutrons, 
which are contained in a neutron source immediately below the ionization 
chamber. The ionization chamber of a neutron well-logging instrument 
is so designed that the instrument will not respond to natural radioactive 
emanations but is sensitive only to secondary-gamma-ray radiations ex- 
cited by the neutron bombardment. Current amplification, transmission . 
to the surface, and surface amplification are similar to the gamma-ray- 
recording process. 

Figure 192 demonstrates a typical field setup. Service equipment 
consists of one truck, called the "hoist truck," which carries a reverse 
concentric cable, the hoist, the power-supply unit, and other equipment 
necessary to the mechanical part of the operations. The second truck, 
lighter in weight and called the "instrument truck," carries the automatic 
recorder, amplifiers, and other electronic equipment used in recording the 
logs. Constant communication between the two trucks on the job is pro- 
vided by an electric intercommunication system. The measuring sheave 
and weight indicator suspended and centered over the hole keep the hoist 
operator informed as to well conditions and the position of the subsurface 
instrument. Depth of the instrument as measured by the calibrated sheave 
is indicated electrically on the odometer dial in the hoist operator's control 
panel, on a similar odometer in the instrument truck, and on the record- 
ing paper. 

Figure 193 is an illustration of the interior of an instrument truck. 

Interpretation 

Any well log must be interpreted in terms of geology and strati- 
graphy before its utility can be realized. Because the proper interpretation 
of a radioactivity well log will identify the various formations represented 
on the log and determine their characteristics and extent, such interpreta- 
tion necessarily involves a wide variety of both geology and bore-hole 



lANE-WElLS 
WEIGHT INDICATOR 




Figure 192. Typical field setup for radioactive well logging. 



424 



Subsurface Geologic Methods 



conditions. The fact that these factors do vary widely from well to well 
and from field to field requires that theory be supplemented by wide 
experience and empirical knowledge to derive the most reliable log inter- 
pretation. The interpretation data that follow represent a summary of 
extensive rJadioactivity-well-logging experience. They are based on the 




Figure 193. Interior of an instrument truck. 



behavior of gamma-ray and neutron curves under the geologic and bore- 
hole conditions most commonly observed. An important part of the 
correct interpretation of any log is the accurate determination of the tops 
and bottoms of formations. This procedure requires particular attention 
for radioactivity well logs because of the characteristically sloping transi- 
tions, which represent formation contacts. 

Figure 194, which is a portion of a gamma-ray curve, represents the 
correct point for determining the top and bottom of a zone. In all cases, 
regardless of the magnitude of the break, the midpoint or center of a mini- 
mum-maximum intensity value on the curve will most reliably indicate 
the actual formation contact. Shown previously in figure 190 is the 
relative radioactivity range of the rocks encountered in oil-well logging. 
A working knowledge of the local stratigraphy is necessary for the cor- 
rect geologic interpretation of the gamma-ray curve. 



Subsurface Logging Methods 425 

Table 22 is a laboratory analysis of several hundred rock samples 
showing the average radioactivity in radium equivalents per gram of 
rock. This table is taken from "The Total Gamma Ray Activity of Sedi- 
mentary Rocks as Indicated by Geiger Counter Determinations" by Rus- 
sell,^^ who has pointed out that an increase in radioactivity is directly 
proportional to an increase in shale or silt in the strata. Furthermore, 
the shade or color of the rock has a relationship to the amount of radio- 



5000 


1 


20 


SHALE 


} 




30 




J 






'' 


40 


V. SAND 
<^ OR 


TRUE THICKNESS 
OF SAND OR IIME 




5050 


C LIME 






60 


\ 


,, 






■J 


70 




^ 


1 


^ 



Figure 194. Showing method of determining true thickness of sandstone or limestone 
using midpoint of transition for top and bottom of zone. 

activity. The darker the shade or color, the higher the radioactivity, ex- 
cept for rocks stained by oil or asphalt. One notable exception is coal, 
which is very low in radioactivity. For general interpretation purposes, 
we can conclude that formations encountered during a radioactivity survey 
will be of the value shown in table 22.^^ Two things must be kept in mind, 
however. The first is that a knowledge of the local stratigraphy is im- 
perative for the correct interpretation because sandstone, limestone, and 
dolomite have so nearly the same value that they cannot be accurately 
differentiated by gamma-ray evaluation alone. The second is that in some 
areas of complex geology some sandstone and/or limestone have been 
encountered that are as high in radioactivity material as the usual shale. 
This phenomenon has occurred only in some Gulf Coast and California 
areas; none has been logged in Midcontinent fields. 



-^ Russell, W. L., The Total Gamma Ray Activity of Sedimentary Rocks as Indicated by Geiger Counter 
Determinations: Geophysics, vol. 9, no. 2, Apr. 1944. 
^ Russell, W. L., op. cit. 



426 



Subsurface Geologic Methods 



Neutron-curve interpretation is a matter entirely different from 
gamma-ray interpretation. It has been stated previously that the response 
of the instrument is controlled mostly by hydrogen. Response of the 
instrument opposite hydrogen is very low. It matters not in what form 
the hydrogen occurs, whether in gas, oil, water, or shale, the curve is 
characterized by a low reading on the graph or to the left of the chart. 
For this reason a neutron curve used alone cannot be interpreted further 



TABLE 22 
The Relationships of Various Rock Types to Radioactive Values 



Lithologic type 




Average 
radioactivity 

in radium 
equivalents 

per gram 
XlO-12 



1. 

2. 
3. 
4. 
5. 
6. 
7. 

8. 

9. 
10. 
11. 
12. 
13. 
14. 
15. 

16. 
17. 
18. 

19. 
20. 

21. 
22. 



23. 



Black and grayish-black shale 

Dark to black shales, neither calcareous nor sandy 

Shales including sandy shales 

Marls and limy shales, grayish-black and black 

Sand and shale 

Dark to black shales, not calcareous, but sandy 

Medium- to light-gray shales, neither sandy nor 
calcareous 

Siltstone 

Medium- to light-gray shales, not calcareous, but sandy 

Marls and limy shales, dark 

Sandstones, silty but not shaly 

Shaly sandstones 

Marls and limy shales of light shades 

All sandstones, including shaly sandstones 

All sandstones, excluding shaly sandstones, but in- 
cluding silty types 

Sandstones free from silt and shale 

Shale-free limestones and dolomites 

Microcrystalline to earthy limestones and dolomites 
of medium to light shade 

Medium- to light-shade, shale-free limestone 

Finely to coarsely crystalline limestones and dolo- 
mites of medium to light shade 

Medium to light shade, shale-free dolomite 

Effect of shade in shale-free limestone and dolomite: 

A. Light gray to white 

B. Medium shade 

C. Dark to black 

Estimated original permeability of sandstone before 

cementation: 

Very high 

High 

Low 

Very low 



40 

74 

164 

3 

9 

16 

17 
11 
18 
10 
26 
40 
16 
131 

105 
76 
64 

28 
33 

24 
21 

30 
22 
10 



35 

37 
40 
24 



26.1 
22.4 
16.2 
16.5 
13.5 
13.2 

11.3 
10.3 
9.0 
8.8 
7.3 
7.0 
6.8 
5.3 

4.0 
4.1 
4.1 

4.0 
3.8 

3.1 
3.1 

3.1 
4.1 
6.1 



2.9 
5.1 
6.6 
7.5 



Subsurface Logging Methods 



427 



than to show the presence or absence of hydrogen. However, the neutron 
curve used in combination with the gamma-ray curve can be interpreted 
to locate possible porous zones in producing strata very accurately. 

In figure 195 a portion of a combination radioactivity log is shown 
with the gamma-ray curve to the left, indicating a productive limestone 
of considerable extent. The tops and the bottoms of the formations have 



RADIOACTIVITY LOG 



GAMMA-RAY CURVE 
RADIOACTIVITY INCREASES 



NEUTRON CURVE 
RADIOACTIVITY INCREASES 




INDICATES POSSIBLE 
POROUS ZONES. 



Figure 195. Combination radioactivity log indicating productive limestone of con- 
siderable extent. 

been determined as previously explained by selecting the midpoint of 
the transition. The neutron curve shown on the right is also interpreted 
by selecting midpoints on the transitions. "Throws" to the right are con- 
sidered barren or impervious strata, while "throws" to the left indicating 
the presence of hydrogen are considered to have fluid in place and are 
therefore interpreted as possible porous zones. 

Applications 

The radioactivity well log was originally developed with the thought 
that some direct relationship might exist between natural radioactivity 
and the presence of petroleum. Experience to date indicates the absence 
of any such relationship. However, the indirect applications of the radio- 
activity well log to the location of petroleum reserves are numerous and 
varied. 

The first advantage of the radioactivity well log over any other type 



Radioactivity Log Electrical Log 

Gamma Ray Resistivity 




Figure 196. Comparison between a gamma- 
ray curve and an electrical-resistivity 
profile taken four years previously in 
same -well, Rincon area, California. (Lane- 
WeUs.) 



Subsurface Logging Methods 



429 



of well-logging method is its ability to log through steel casing. This 
permits the stratigraphic study of old wells drilled prior to the develop- 
ment of geophysical well-logging methods now in common use. The loca- 
tion of upper cased-off potential producing zones has presented a logical 
field for this type of logging, particularly where no information was 
available or where the available information was doubtful. 

Variations of this primary application are apparent. The location 



6200 



RUN He. ) UFORE ! 




Figure 197. Location of camotite (radioactive) cement with gamma-ray curve. 

(Lane-WeUs.) 



of the top of the producing zone for bottom-water shutoff, the correction 
of some of the earlier drillers' logs, the location of upper potential fresh- 
water sands for salt-water disposal, the supplying of additional infor- 
mation where cores were not completely recovered or were lost, and the 
location of the top and bottom of an oil-producing zone for gas-oil-ratio 
control are all applications of this type. 

Many logs are run for measurement checks of various kinds. Drill- 
pipe measure, casing measure, and electric-log measure often disagree. 
On many of the older wells the original zero point has been lost. On 
other wells, both old and new, a new bottom has been established. Many 
of the earlier sample logs did not take into consideration the time lag 
between the point of origin of the sample and the depth of the well at 



430 



Subsurface Geologic Methods 



the time the sample was caught. Because of the tested accuracy of the 
measuring system employed and the ability of the instruments to indicate 
the changes in formations behind casing, radioactivity logs have become 
a popular means of resolving all manner of measuring discrepancies. 

An additional feature of radioactivity well logging is the collar log, 




Figure 198. Radioactive curves in Branyon field, Caldwell County, Texas. Radio- 
activity increases from left to right. (Lane-Wells.) 



Subsurface Logging Methods 



431 



which is a record of the exact location of each casing collar. It is re- 
corded electrically by means of a collar locator which is a component 
part of the gamma-ray instrument. Thus, the collar log is recorded simul- 
taneously and on the same chart with the gamma-ray curve. This provides 
a permanent record of the fixed relationship of collars to formations. 




Figure 199. Radioactivity curves in Salt Flat field, Texas. Radioactivity increases 
from left to right. Note maximum point on gamma curve opposite Eagle Ford 
shale. (Lane-Wells.) 

Formation tops and bottoms are accurately established in relation to 
the nearest casing collar. This combination eliminates the many measur- 
ing discrepancies that may occur in various means of well measuring. 
The second advantage of radioactivity well logging is its ability to 
log in contaminated well fluids. In areas such as central Kansas where 
salt beds are encountered in drilling, the resistivity of the drilling fluid 



432 



Subsurface Geologic Methods 



is lowered to a point where it is impossible to get a good electric log. 
The high resistivity of fresh-water muds presents a similar problem in 
attempts to log fresh-water sands electrically. Oil-base mud is used in 
some areas to eliminate the infiltration where the productive zones are 
partly depleted or where bottom-hole pressures are low. Since oil is 



GAMMA-RAY CURVE 

RADIOACTIVITY INCREASES 



NEUTRON CURVE 

RADIOACTIVITY 
INCREASES ^ 




Figure 200. Characteristics of gamma and neutron curves in Cushing-Drumright field, 
Oklalioma. This well is 20 years old. (Lane-Wells.) 

a dielectric substance, it shows a high electric resistance, which makes 
it difficult to obtain satisfactory electric logs. Electric logs have been 
run in oil-base mud by using electrodes that make a sliding contact to the 
walls of the well. However, washed-out places and irregularities in the 
well bore tend to defeat the purpose of the contact electrode and cause a 
curve to be produced that is sometimes difficult to interpret. Since the 



Subsurface Logging Methods 



433 



radioactivity well log is a measurement of radioactive emanations, it is 
not affected by salt-water mud, fresh-water mud, oil-base mud, or other 
contaminated fluids. 

Radioactivity well logs are easily correlated with other types of 
geophysical well logs and such information obtained from well bores 
as sample logs and core analysis. Radioactivity logs and the types of 
electric surveys in common use today are easily correlated. Although 
there is no relationship between the two types of surveys and they are 



Well A 



V/ell B 



Well C 



Gamma Ray 



Gamma Ray 




Figure 201. Correlation of radioactivity logs made in cased wells along a two-mile 
strike section, Long Beach field, California. (Lane-WeUs.) 

often surveyed under different well conditions or years apart, there still 
exists a similarity in response that permits accurate correlation. This is 
most fortunate, for it is simple logic to comprehend that a multiconductor 
cable such as is employed in electric logging used under varying fluid 
and mud conditions cannot be expected to measure with complete accuracy. 
Figure 203 illustrates the typical response of radioactivity and elec- 
tric logs to the usual formations encountered in oil-well drilling. These 
generalized curves are shown to illustrate the most typical response of 
radioactivity and electric logs to the various types of formations. These 
typical curves should not be used as a criterion for analysis of any par- 
ticular log since in practice a wide range of response may exist. In making 
correlations between radioactivity and electric logs, the gamma-ray curve 
is correlated with the self-potential curve, while the neutron curve is 
compared with the shallow-resistivity curve. 



434 



Subsurface Geologic Methods 



Radioactivity well logs are also employed for a variety of specialized 
applications. One of these is the logging of added radioactive material. 
In the study of squeeze cement jobs on the Gulf Coast, it has been found 
desirable to know the vertical travel of the cement and its mass distri- 
bution behind the casing string. By means of a radioactive tracer such 
as carnotite mixed with cement, it is possible to determine the direction 
and extent of the cement travel. The usual procedure is to record a 
gamma-ray curve of the well under normal conditions. The mixture of 



GAAAMA RAY 
CURVE 



GAMMA RAY 
CURVE 



AVERAGING NATURAL 
RESISTIVITY POTENTIAL 
CURVE CURVE 

WEIL 3 




Figure 202. Gamma ray-electrical log correlation of section in Long Beach field, 

California. (Lane-Wells.) 




a ^ 
in v 



436 Subsurface Geologic Methods 

carnotite and cement is then squeezed. After the cement has set and the 
plug has been drilled, a second gamma-ray curve is run on the same 
sensitivity as the first. The difference in gamma-ray intensity between the 
first and second runs is caused by the carnotite. Thus, by comparison of 
the two curves, the top and bottom of the cement travel can be determined. 
The same technique applies to squeezing of plastic material. 

Another unusual application for radioactivity well logs is in the 
location of permeable zones in oil-producing horizons. The combination 
radioactivity log is run through the section to be studied. The gamma-ray 
curve provides the local stratigraphy, and the neutron curve indicates 
the possible fluid-bearing zones. A quantity of oil or water containing 
a radioactive tracer is then spotted opposite the section to be studied. 
Pump pressure is applied to force the radioactivity fluid into the produc- 
ing horizon. A second gamma-ray curve is then run on the same sensitivity 
as the original run. The presence of the radioactive tracer in the zone 
increases the radioactive valuation of the zones and the relative permeabil- 
ities of the zones can be estimated by a comparison of the two gamma-ray 
curves. 

General Considerations 

Many varied methods for the logging of oil wells have been de- 
veloped and practiced through the years since exploration for oil began, 
the most common being the use of a sample log obtained by geologists, 
who examine and identify the drill cuttings from the well and plot their 
findings on a strip of paper with relation to depth. As the science of 
oil exploration developed, so too did the science of oil-well logging 
advance, until today the oil operator can call upon approximately eleven 
general methods. Although these many methods could be used to log oil 
wells, only a few are being used extensively at the present. The methods 
most frequently employed commercially are the following: 

1. Optical 

(a) Examination of samples 
(h) Fluorescence 

2. Mechanical 

(a) Drilling rate for hardness 

(h) Drilling reaction, hardness, texture 

(c) Hole calipering 

3. Radioactivity 

(a) Natural, radioactive-substance content 
(h) Artificial, hydrogen content 

4. Electric 

(a) Natural potential 
(h) Formation conductivity 
(c) Mud conductivity 
In the selection of the best method for the logging of an oil well, 



Subsurface Logging Methods 437 

consideration must be given to the problems to be solved, the methods 
available, and the adaptability of the methods and their cost. The method 
selected should, therefore, have as many as possible of the following 
properties: 

1. Significant relationship to the lithology of the rocks. 

2. Detection of oil, gas, or certain minerals penetrated by the bore. 

3. Many vertical variations, ability to differentiate strata. 

4. Persistence of vertical variations laterally, to provide good sub- 
surface correlations. 

5. Ability to work under almost any bore-hole condition. 

(a) Cased holes, dry or filled with fluid 

(b) Open holes, dry 

(c) Open holes filled with mud or water 

(d) Open holes filled with oil or gas 

6. Location of porous strata. 

7. Economy of operation. 

8. Simplicity of interpretation. 

Radioactivity well logs fit nearly all of these requirements. Coring 
and core analysis constitute the only positive means of lithologic identi- 
fication, but owing to excessive cost the method is not very widely em- 
ployed. The relationship of the radioactive content of the strata to lith- 
ology has already been pointed out, and a suitable lithologic identification 
can be executed by the gamma-ray curve. 

No positive means of the detection of either oil or gas has yet been 
determined other than fluorescent and chemical examination of samples 
and cores. At present the combination radioactivity log makes no claim 
to the direct detection of oil or gas, but the indirect use of the log by 
correlation or structural determination has been the means of locating 
many new pay horizons. It is therefore stated that radioactivity logs are 
as capable in the detection of oil or gas as other logging methods, with 
the exception of visual examination of conventional cores. Research and 
future development may well provide a sure means of oil detection, 
either through the development of an additional curve or through the 
better interpretation of analysis. The requirements 3 and 4 given above, 
vertical variations for differentiating strata and vertical variations later- 
ally of ample magnitude to provide subsurface correlations, are ade- 
quately filled by the radioactivity well log. Abnormally high radioactive 
shales in most areas serve as base markers for the long-range correlation 
of gamma-ray curves. Within confined areas it has been found that the 
neutron curve correlates readily, even to the extent that the evaluation of 
potential reserves is possible. 

In considering requirement 5, the advantages and the flexibility of the 
radioactivity log are most obvious. The radioactivity log can be obtained 
in cased holes, dry or containing bore-hole fluid of any type, and in open 



RADIATION INTENSITY INCREASES 



flUtO lUIMC sano 

nOn riuiD •CAliHC lua 



ANO OENSC SANO Ol lIMt 
IHAIC 

iKAtT UNO 01 IMl 

9HALI 

AND OINU lANO Ol lIMl 



cdadinc to s 




SHAll AP^tAlS AS NOIMAt (HAll 



• EAtINC ON ftOnOM 



TOIITI 

SHAtE PAlUf waSHIO Oin 

Ol tiMf. DENSE ON TOr ftU'O ON lonoM 

SHA(| 

IME CIADINC 
}l liMf DENSI 



HERENCIATEO SHAIE SANO AnO I'Mt OiNSt 



IMC riuio spomo 

DENSE 



Figure 204. Showing correlation of radioactivity-log profile with various 

rock types. 



Subsurface Logging Methods 439 

holes, dry, producing gas, containing salty mud, conditioned mud, fresh 
water, oil, oil-base muds, or any mixture of these fluids. 

The location of porous strata by the neutron curve, especially when 
it is used simultaneously with the gamma-ray curve, has been described 
and explained. The ability to determine porous strata by radioactivity 
means has been proved beyond all doubt by the hundreds of successful 
completions of oil wells in which radioactivity logs were employed. 

Because of the flexibility of the radioactivity log it becomes also 
the most economical survey available. No special arrangements or hole- 
conditioning procedures are necessary. 

Simplicity of interpretation is one of the primary advantages of 
logging by radioactivity means. It is not necessary for the oil operator, 
engineer, or geologist also to become a. nuclear physicist. A very basic 
understanding of the principle upon which a radioactivity log operates, 
the action behavior of the recorded picture obtained, and a knowledge 
of the stratigraphy of the region are all that are necessary for accurate 
interpretation. 



CALIPER AND TEMPERATURE LOGGING 
WILFRED TAPPER 

The uses of caliper and temperature records are sometimes so inter- 
related that a discussion of one log presupposes a discussion of the other. 
Here, however, for purposes of clarity, the records and the tools used 
to obtain them are treated separately. 

It is the purpose of this section to give an outline of the history, 
development, construction, and uses of caliper and temperature electrodes. 
A knowledge of the physical construction of both types of electrodes 
results in a clearer understanding of the data obtained and more efficient 
utilization of the logs. 

The few anomalies cited as examples do not pretend to be compre- 
hensive. The uses for the various logs listed surely do not exhaust present 
or future possibilities in the oil industry or in other fields. 

The writer is indebted to Mr. H. K. McArthur, Mr. J. K. Reynolds, 
and Mr. W. D. Owsley, of the Halliburton Oil Well Cementing Company, 
for advice and criticism in the preparation of this section. 

Caliper Logging 

Even in the early days of cable-tool drilling, oil men were well aware 
that drill holes did not stand true to gauge. As holes were drilled deeper 
and exploration moved southward to younger sediments, this fact became 
painfully obvious. 

One of the factors influencing the development and use of the rotary 
drill was its ability to cut through young, unconsolidated sediments and 



HOLE DIAMETER (inches) 
10 20 30 . 



HOLE DIAMETER (inches) 
10 20 30 



1 


























\ 






s 








1 


1 














1 


j 








1 






























j 


\ 














1 


/ 








' 
























1 








I 








\ 








/ 






1 


\ 








i 






















1 

















WATER BASE MUD OIL BASE MUD 

Figure 205. Caliper logs of two wells in 
Merey field, Texas, showing influence of 
type of mud on caving. 



Subsurface Logging Methods 441 

still have the bore hole tend to stand up. Nevertheless, numerous prob- 
lems connected with rotary drilling and oil producing were a direct result 
of hole caving in rotary holes. The exact nature and extent of this caving 
was unknown. Every driller and every geologist had a theory, but there 
was no exact knowledge. Subsurface bore-hole caving was likened to 
surface erosion, but owing to the different character of the sediments and 
the greatly accelerated subsurface erosional forces, this parallel could 
not be drawn too far. 

Present knowledge indicates three major reasons for differential hole 
size in a hole drilled with rotary tools: (1) action of the drilling fluid, 
(2) action of the bit, and (3) action of the drill pipe. Of these, it is 
believed that the action of the drilling fluid is most important. 

The hole change caused by mud or any conventional drilling fluid is 
due either to a chemical effect (hydration) or a mechanical effect (attri- 
tion or dissolution) . Of these two, it is believed that the former is the 
more important cause. 

Water-base muds must have a tendency to cause many shales to swell 
and heave or to disintegrate. Many shales disintegrate beyond the 32- 
inch range of the modern caliper tool. Interestingly enough, a mud cake 
may be built up on the face of a formation, causing a hole to caliper 
smaller than bit size. 

In order to reduce hole change, many muds other than water-base 
muds are used. Oil-base, oil-emulsion, silicate-base, and salt-base muds 
are all commonly used to prevent the caving of shales or soluble forma- 
tions. It is not the purpose of this section to discuss the merits of various 
types of muds. The effect of these muds on hole size has been clearly 
shown to the industry by caliper logs. Figure 205 shows caliper logs on 
offset wells, one drilled with oil-base mud and one with water-base mud. 

In 1932 M. M. Kinley made a successful attempt to caliper a hole. 
A few years later R. B. Bossier measured the enlargement of a hole caused 
by shooting. These early tools measured a short section of the well and 
were used mostly in shallow areas. The original Kinley caliper had four 
separately actuated arms, each arm giving a separate record. A stylus- 
recorded strip chart was obtained. 

The present modern caliper used by the oil industry was developed 
from the original M. M. Kinley tool by the Halliburton Oil Well Cement- 
ing Company in 1940. 

The present caliper (figs. 206 and 207) is a chrome-plated tool three 
inches in diameter and approximately five feet long. It consists of an 
oil-filled chamber containing the electrical components, the four caliper 
arms, and the releasing mechanism. The tool itself is standard equipment 
on an electrical-service truck and is run in a well on a five-sixteenths-inch 
logging line. 

The four spring-actuated arms of the tool contact the walls of the 
bore hole when they are released. The motion of these arms is trans- 



442 



Subsurface Geologic Methods 



mitted to a rheostat inside the oil-filled chamber by means of a flexible 
bronze cable-and-pulley system in such a manner that the change in re- 
sistance of the rheostat is always proportional to the change in average 
diameter as measured by the four arms. Owing to the spring tension in 
the arms, the tool will be approximately centered in the well, unless the 
hole is considerably off vertical. 

The arms are held in a closed position by a steel band when going 
in the hole. This band can be broken at will, either by firing a brass 
projectile located underneath or by spudding on bottom. 

The chamber of the tool is filled with oil and kept hydrostatically 




Figure 206. Caliper in open position. 



Figure 207. Caliper 
in closed position. 



Subsurface Logging Methods 



443 



balanced by means of a large rubber diaphragm, which acts as a volume 
equalizer between the oil and the mud, compensating for the difference 
caused by the motion of the push rods and by changes in temperature. 

A constant direct current is supplied to the rheostat in the tool, and 
the resultant potential drop across it is measured and recorded as a caliper 
log. These logs take the form of a continuous galvanometer trace recorded 
on film showing the average diameter of the bore hole, recorded as a func- 
tion of depth. 

A study of numerous caliper logs soon leads one to the conclusion 
that caving patterns exist. Certain generalizations may be made as to the 
relative ability of rocks to stand up to bit size. These generalizations are 
shown in graphic form in table 23. 



TABLE 23 
Ability of Rocks to Stand Up to Bit Size 



Rock 


Poor 


Medium 


Good 


Sand 

Shale 

Chalk 

Limestone 

Dolomite 

Anhydrite ... 

Salt 


X 
X 


X 


X 

X 
X 
X 
X 



Although similar drilling conditions and similar muds tend to stand- 
ardize caliper logs, some astonishing long-range correlations may be 
made by using them, even for wells drilled under entirely different cir- 
cumstances. In the west Texas area, a number of horizons may be recog- 
nized on caliper logs, from Upton County, Texas, to the Hobbs field. 
Lea County, New Mexico, a distance of 120 miles. Here caliper-log cor- 
relations are more trustworthy than those made with electric logs. 

The most obvious use for the caliper log in the oil industry is as a 
tool to calculate the proper amount of cement necessary to fill up the 
annular space between the casing and the open hole to a desired point. 
The actual amount of cement necessary for a desired fill is often two or 
three times the amount one would use from theoretical calculations. 
Figure 208 is an actual caliper log of a well in Smith County, Texas. The 
hole was drilled with an 8f-inch bit, with S^-inch casing set on bottom. 
Theoretical fill-up is 22 sacks of cement per 100 feet of hole. For more 
than 1000 feet of section 220 sacks of cement is the theoretical amount 
necessary to fill back to 9500 feet. The actual amount of cement needed 
is 544.4 sacks, or more than twice the theoretical quantity. 

Another problem encountered in the successful completion of an oil 
well is the location and reaming of tight spots in the hole, so that casing 



444 Subsurface Geologic Methods 

can be set and successfully cemented without channeling and bridging. The 
value of a caliper log in locating these zones has been demonstrated many 
times. 

Modern drilling practices presuppose the use of many scratchers, 
centralizers, and guides welded to the casing to assist in obtaining a 
better cement job. All these tools have proved extremely useful in obtain- 
ing better cementing jobs and eliminating costly squeeze jobs, but they 
are useless unless properly positioned in the hole with the aid of a caliper 
log. 

In plugging back with cement, plastics, or gravel packing, a knowl- 
edge of hole size is invaluable. In remedial work of this kind, well records 
are usually inaccurate or nonexistent, and it is only through an application 
of the knowledge gained from a caliper log that a successful job may be 
performed. 

In the field of drill-stem testing, a knowledge of caving conditions 
has saved oil operators an untold amount of money. Figure 209 is a 
caliper log of a well in Smith County, Texas, where the operator wished 
to find a packer seat to drill-stem test the Paluxey sand at 7140 feet. A 
glance at the log will show that, using trial-and-error methods, the chances 
of successfully setting a packer are relatively small. By applying the 
information to be gained from the use of a caliper log, the proper point 
to set the packer and the proper size of packer are easily determined. 

Many other uses have been found for caliper logs. The successful 
completion of a fishing job may depend on a knowledge of the size of the 
hole above the junk. A log made after a fishing job is completed will pro- 
vide the necessary data successfully to resume drilling operations. 

A knowledge of hole size is essential in evaluating an acidizing job, 
picking a zone to side-wall-core, evaluating the results of shooting with 
nitroglycerine, and finding a proper zone to gun-perforate. The present 
caliper log has fulfilled all these functions. 

Temperature Logging 

As early as 1869 Lord Kelvin conducted experiments in measuring 
earth temperatures at a depth of 350 feet in the ground. Since then, 
geologists have speculated on the geothermal gradient in the earth's crust. 
Even with such an early start, little has been done in the way of quanti- 
tative work with earth-thermal measurements. 

At the present time thermal measurements in either a cased or open 
hole are usually obtained by means of a continuous-recording, extremely 
accurate, electronic thermometer. Such a tool is standard equipment on 
an electric-logging truck and is run on a five-sixteenth-inch conductor 
cable. 

The temperature electrode is a three-inch rubber-covered tool about 
six feet long. In a groove in the rubber coating of the electrode is a 
twenty-inch length of platinum wire, which is exposed to the mud column. 



9300. 



0" 10" 20" 30' 



9400_ 



?500_ 



9600. 



9700 _ 



9800 



9900. 



10000. 



10100. 



10200. 



10300^ 



10400. 



-32.6 sks 



-56.4 sks- 



-82.8 sks- 



-123 sks. 



-90.1 sks 



-60sks- 



-33.2 ; 



-28.1 sks. 



-25.2 sks- 



— 23 sks. 



Il0500. 



Figure 208. Caliper log of 
well in Smith County, 
Texas, showing effect of 
hole size on cement cal- 
culations. 



446 Subsurface Geologic Methods 

This wire is small in diameter and assumes the temperature of the fluid 
around it rapidly. Changes in temperature produce changes in the resis- 
tance of the wire, which are detected by a bridge circuit in the electrode. 
Alternating current from a 500-cycle generator is supplied to the bridge 
terminals. The signal terminals of the bridge are transformer-coupled 
to the grid-cathode circuit of an a.c. amplifier circuit in the tool. The 
amplified a.c. signal is rectified and sent to the surface as a d.c. signal, 
where it is calibrated in degrees. In order to buck out the static value of 
this signal, a known matched signal of opposite polarity is placed in 
series with the electrode signal. The resultant d.c. signal is amplified in the 
instrument tray and recorded in the camera. A switch is provided in the 
tool for changing signal points at the a.c. bridge, so that the bridge can 
always be operated close to the balance point. This is necessary for two 
reasons, to reduce noise and to keep the electrode system from being 
saturated. The resultant log comes as a plot of temperature versus depth. 

The standard electrode may be obtained in two temperature ranges, 
from 20° F. to 280° F. and from 60° F. to 340° F. Tools capable of 
being run in tubing are also made. 

A fundamental knowledge of temperature gradients and temperature 
anomalies, as expressed in drilled holes, is necessary before the myriad 
uses of temperature logs can be fully realized. 

Measurements made by a thermometer lowered in a drill hole give 
the temperature of the drilling fluid. Unless the hole has not been cir- 
culated for several weeks, the temperature of the mud is very different 
from that of the formations. The mud is usually colder at the bottom 
and hotter at the top of the hole than is the surrounding strata. Thus, 
when circulation is stopped, the mud will warm up in the lower part of 
the hole and cool at the top. The speed of this heat exchange will depend 
on the lithology of the bore hole. 

To illustrate this, take a well 8000 feet deep, as illustrated in figure 
210. The geothermal gradient can be represented by a straight line, as 
shown in curve 1. The temperature of the mud, when circulation ceases, 
is almost the same from top to bottom, as indicated by curve 2. The 
temperature gradients cross at point A. 

If the well is left idle for several days, the temperatures will tend 
to equalize and the mud curve will tend to rotate from 2 to 1 about the 
axis A. 

The cooling or warming of the mud at a certain depth will depend 
on the thermal conductivity of the formations and the size of the hole. 
Experience has shown that equilibrium is reached sooner opposite sands 
than shales. This can be explained by the fact that (1) hole size is smaller 
opposite sands than shales, and, therefore, the volume of the mud is less, 
and (2) the thermal conductivity of sands is greater than that of shale. 

Thus it will be seen that during thermal evolution, sands will exhibit 
a lesser temperature than adjacent shales in the top part of the hole and a 



10- 



20 



30 



70QQ 



A£L 



_2Q. 



3SL 



i 



AQ. 



7QSQ 



jiiL 



J£L 



3SL 



90 



7100 



10 



20. 



-20. 



AQ. 



i5Q_ 



J5Q. 



2£L 



Figure 209. Caliper log in Smith County, Texas, used to find packer seat. 



448 



Subsurface Geologic Methods 



higher temperature in the bottom part. Curve 3 of figure 210 represents 
the temperature of mud about ten hours after circulation and illustrates 
this point. 

If a temperature record is obtained immediately after circulation, a 
flat curve resembling curve 2 of figure 210 will result. The same type of 
curve will be obtained after several days have passed and equilbrium has 



0' 




50" 


100° 


ISO** 


200** 




~ ■:- r. 


\ 


[ 1 












shale 




^® 


\ 


\ 












sand 




s=?± 


-\ 


S 












2000' 




^s 


> 


^ 












shale 




^fi 




1 


A- 










4000' 




pK 




\ 












sand 




?sr 








i\ 










shale 




IS 






1 \ 










600d 




=r^ 






1 \ 












K 






1 


\ 


\ 






sand 






— _ 





._!__ 


-^ 


-iN 


\ 




8000' 






-2-1 


-A 



Figure 210. Chart showing temperature gradients. 

been reached. The most pronounced anomalies are obtained 24 to 36 
hours after circulation has ceased. 

Although from the foregoing discussion one can see that it is possible 
to distinguish sands from shales, the temperature log will not yet replace 
the electric log. Factors such as chemical reaction, changes in hole size, 
the movement of fluids between sands, and the movement of hydrocarbons 
can alter the temperature in a well. 

Thus far only open-hole-temperature measurements have been con- 
sidered, but the presence of a string of casing does not disturb the thermic 



Subsurface Logging Methods 449 

state of a bore hole. Therefore, all of this previous discussion applies to 
cased holes when they have been cemented, provided the temperature log 
is made four or five days after cementing. 

Cement generates considerable heat as it sets up, and this factor has 
resulted in the principal application of temperature logs: The determina- 
tion of a cement top behind casing by means of thermal measurements. 
The magnitude of this temperature increase varies with the time elapsed 
since cementing and the quantity of cement used. 

Most of the heat is generated a few hours after the cement job has 
been completed. After this interval it is quite possible that the forma- 
tion will absorb heat faster than it is generated by the setting cement, 
thereby cooling the mud. As a rule the best time to run a temperature 
survey is from four to eighteen hours after the cementing plug has hit 
bottom. The exact interval depends on a number of factors. Most oper- 
ators prefer not to release the pipe pressure until after the initial set of 
the cement, which is a function of the type of cement, tlie type of water, 
the temperature, the pressure, and other variables. 

The quantity of cement also affects the magnitude of a temperature 
anomaly. The amount of deflection tends to vary as does the amount of 
cement, which is a function of hole size. The joint use of a caliper log 
when trying to interpret a temperature log is often useful in this respect. 

Several precautions should be taken when obtaining temperature 
logs in a cased hole. Circulation after cementing has been completed 
results in the heat evolved by setting cement being dissipated, and a 
trustworthy record is not obtained. 

Temperature surveys in their present form will tell how high cement 
is in the annular space and, comparatively, how much cement is behind 
pipe. However, the survey does not indicate where the cement is. It is, 
for all practical purposes, impossible to detect channeling on a cement 
job by means of a temperature survey. 

Numerous other uses have been made of temperature records in both 
cased and uncased holes. As gas enters a well, either through a hole in 
the casing or into a bore hole on an uncased well, it expands and cools. 
This characteristic has been used to find oil-gas contacts or a hole in the 
casing. However, any use of a temperature log presupposes that the tem- 
perature anomalies being measured are of a greater magnitude than four 
or five degrees, these being the order of formation thermal differences. 

WELL LOGGING BY DRILLING-MUD AND CUTTINGS ANALYSIS 

ARTHUR LANGTON 
Today the determination of the fluid content of the porous forma- 
tions penetrated by the bit so that no oil- or gas-bearing formations will 
be overlooked constitutes one of the most important problems in drilling 
for oil and gas. Modern mud-analysis, well-logging equipment, and im- 



450 



Subsurface Geologic Methods 



proved technique allow for the detection of the most minute quantities 
of oil and gas and the exact placing of these shows at the proper depth. 

The Baroid well-logging service has been developed using the fol- 
lowing facts as a basis: In drilling a well the bit disintegrates a 
cylindrical section of the formation. If the pore spaces of this cylinder 
contain oil or gas, part of these fluids will be entrained by the drilling 
mud and part will be retained on or in the cuttings. If the drilling 
fluid and the cuttings are continuously tested on their return to the sur- 
face and the results of these tests correlated with the depth of origin, the 
presence or absence of oil or gas in specific formations can be determined. 

In the application of the method continuous tests are made on the 




i'lGURE 211. Baroid mud-analysis well-logging unit on location. 



mud and cuttings returning to the surface. The test for gas in the mud 
is made by diverting a portion of the circulating mud from the flow 
line to a separator or gas trap, where the mud is thoroughly mixed with 
air and a portion of the gas in the mud is removed. A stream of air 
is drawn countercurrent to the flow of mud in the gas trap, thus mate- 
rially assisting the separation of the gas from the mud. The air-gas 
mixture is then drawn into a "hot-wire" gas-detector instrument, where 
the percentage of combustible gas is determined. The amount of methane 
gas present in the total is determined by controlling the temperature of 
the filament in the gas detector. 

Thus, the gas readings in the mud are recorded as total gas, which 



Subsurface Logging Methods 



451 



includes all the combustible gases and methane gas. This "breakdown" 
of gas readings is made because recent studies have indicated that, with 
few exceptions, all productive horizons logged by the mud-analysis 
method have shown a definite increase of methane gas. Therefore, in- 
clusion of the methane curve on the new log is important to operators, 
since, normally, those zones that do not show that an increase of methane 
may be condemned. 

The test for gas in the cuttings is made by placing a small sample of 




Figure 212. Operating equipment in Baroid mud-analysis well-logging unit. 



the cuttings with a measured amount of water in a closed-container-type, 
high-speed grinder. After cuttings are ground, the air-gas mixture in the 
container is examined for gas in the same manner as described in the 
preceding paragraph. Here, too, the gas is reported as total gas and 
methane gas. 

The presence of oil in the drilling fluid is detected by a physical 
examination of the drilling mud under ultraviolet light. A sample of the 
mud is treated to reduce the surface tension and gel strength, after which 
it is placed in a viewing box. This box is so constructed that all external 
light is excluded; thus the sample may be subjected selectively to either 



452 



Subsurface Geologic Methods 



ultraviolet or white light. The magnitude of the oil shows in fluid-logging 
units is based on the amounts of observed flourescence of the crude oil. 

Oil in the cuttings is determined by using the same ultraviolet-light 
arrangement. Freshly washed cuttings are placed under the light and 
observed for fluorescence. The cuttings are then treated with various 
leaching agents to see if they will give a "cut." Certain minerals 
fluoresce when subjected to ultraviolet light. This fluorescence can be dis- 
tinguished from crude-oil fluorescence by the fact that mineral fluorescence 
will not give a "cut" when treated with a leaching agent. 




Figure 213. Control panel of Baroid mud-analysis well-logging unit. 

Further, the cuttings are subjected to a detailed examination under the 
microscope, in which the operator records the estimated percentage of 
limestone, sandstone, shale, and anhydrite. Note is also made of in- 
dications of porosity and permeability. 

A depth meter, pump-stroke counter, and pump-rate meter provide 
the necessary data for obtaining a rate-of -penetration curve and for 
properly correlating with depth any shows of oil and gas found in the 
mud and cutting samples at the surface. The log is prepared as a plot 
of the magnitude of the oil and gas shows versus depth. Also given is 
a sand- or lime-index curve showing the relative amounts of either sand 
or lime in the cuttings. This curve is intended to be used as a supplement 



Subsurface Logging Methods 



453 



to the drilling-rate curve. The drilling-rate curve and the sand-index 
curve are used as indications of porosity and permeability values and 
of changing formation. They are also used for correlation between wells 
in the same general area because of characteristic variations in formation 
hardnesses and sand or lime content. 

The well-logging service is used as wells are being drilled, and re- 
sults on any formation are available as soon as the mud used in drilling 
the formation reaches the surface. The system does not interfere with the 
drilling operations in any way, and no changes in ordinary drilling equip- 




FiGURE 214. Special filter press designed to test efficiency of fibrous materials for 
restoring lost circulation. "Core" consists of sized rock. 



ment or procedure are required to make it workable. The logging units 
are in reality mobile field laboratories. In addition to the logging in- 
struments, each unit is equipped with complete mud-testing equipment. 
Tests for mud weight, viscosity, water loss, cake thickness, salinity, and 
other characteristics can be made so that a satisfactory drilling mud may 
be maintained. 

The principal application has been in the drilling of wildcat or 
exploratory wells, vvhere its usefulness is apparent in many phases of the 
well program. Coring can be reduced to a minimum by using mud-analysis 



454 Subsurface Geologic Methods 

logging to choose only those sections to core which contain oil or gas. 
The procedure is to drill ahead until a significant increase in drilling 
rate, "a drilling break" indicating a change to a softer, more-easily-drilled 
formation, is encountered. After two to four feet of the soft formation 
are penetrated, further drilling is suspended until the mud, which was 
exposed to this new zone, and the corresponding cuttings are pumped to 
the surface and samples reach the logging unit for analysis. If oil or 
gas shows are obtained, cores of the formation are taken. If nothing of 
interest to the operator is indicated, drilling is resumed until another 
drilling break is encountered; whereupon, the procedure is repeated. Such 
a program has an operational advantage in that the information is avail- 
able a relatively short time after the zone is penetrated. This program 
allows the operator to core, or, if desired, to make drill-stem tests while 
the effect of contamination by the drilling fluid is at a minimum. 

Baroid Well-Logging Units also contain complete core-analysis equip- 
ment by means of which the operator may make determinations of por- 
osity, permeability, salinity, and oil and water saturations. 

The type of log produced is especially useful in those cases where 
conditions exist which make it difficult to get good electrical logs. These 
conditions may apply to an entire area such as the Permian Basin of West 
Texas. Here it is almost impossible, because of the properties of the lime 
formation and the high-saline content of the drilling muds, to obtain good 
electric logs; whereas, the Baroid log gives the operator accurate informa- 
tion on the oil and gas content of the formations. Similar conditions exist 
when the temperature of the drilling fluid becomes exceedingly high, as 
in some of the deep Louisiana wells, or when the chemical treatment of 
the mud is such that it will interfere with the electrical log. Also to be 
considered are isolated formations which are difficult to interpret by the 
electric log. In this category are such formations as the Eocene Wilcox, 
the Cotton Valley, the Travis Peak, and numerous others. The electric 
logs taken through these sections do not readily indicate the fluid content 
of the formation. With the Baroid log, a positive identification of the 
fluid content is made, thus reducing testing to a minimum, with a resultant 
decrease in time and money expended on the completion of the well. 

In certain sections of the country some exploratory wells are drilled 
without taking any cores. After the electrical log is run, sidewall samples 
are taken to check the possible indications shown on the electric log. In 
this manner, the wells are drilled with a minimum output of time and ex- 
pense, but ever-present is the possibility that a productive sand or lime 
will be missed. This possibility is removed by using the Baroid Well 
Logging service during drilling. This type of logging can be depended 
upon to pick up indications of all possibly productive formations, and 
the fluid content of such formations can be further checked by side-wall 
cores. 

This exploratory method is particularly useful in areas of abnorm- 



Subsurface Logging Methods 455 

ally high pressure, where sloughing shale is encountered, and, in general, 
where coring is difficult or impossible. In cases on record, hole trouble 
developed so seriously that it was impossible to make other surveys, and 
the mud and cuttings-analysis log was the principal, if not the only, source 
of information on the lower section of the hole. An operator desiring to 
quit a well because of severe hole trouble, such as junk, lost circulation, 
or high pressures, may spend tens of thousands of dollars conditioning 
the hole just to run the final electric log over perhaps only a few hundred 
feet drilled since the last log was run. If a Baroid log had been obtained, 
it would have provided reasonable assurance that nothing had been passed, 
and the operator could have plugged and abandoned the hole as economic- 
ally as possible. 

The method does not present a complete subsurface picture. It does 
not give quantitative determinations of the amount of oil and gas occurring 
in the formation, nor does it furnish quantitative information on the 
productivity of the oil and gas horizons. A quantitative estimate is 
prevented by the numerous factors which affect the concentration of 
oil and gas in the mud and the cuttings. Some of these factors are 
the ratio of the volume of formation drilled to the volume of mud used 
to drill it; the flushing action of the drilling fluid, which in itself is 
affected by the mud-filtration characteristics, drilling rate, speed of rota- 
tion of the bit, differential pressure, and effective porosity and perme- 
ability; and the amount of oil and gas which is recirculated, which depends 
on the viscosity and gel properties of the drilling mud. However, the 
method does give reliable qualitative information on the occurrence of 
oil and gas, and the interpretation of the log should be made in the light 
of all available information on that section of the hole logged. It is 
extremely valuable in minimizing coring, as outlined before, in re- 
moving the ever-present possibility that a production zone will be missed 
and in rounding out the picture presented by other formation data. 

DRILLING-TIME LOGGING 

G. FREDERICK SHEPHERD 

Rate of penetration is considered here as the time required to rotary- 
drill a linear unit of depth of a geologic formation of the earth's crust. 
It is believed that drilling-time characteristics constitute a diagnostic prop- 
erty of a rock resulting from its composition, mode of deposition, degree 
of compaction, and other known physical features by definition of which 
the rock is described. At present, drilling-time properties, measurable in 
terms of rate of penetration, are qualitative in scope and relative in their 
interpretation. That they may become quantitative and definable in fixed 
values that may be significant in the determination of lithologic types 
is anticipated. 

Rate of penetration may be measured in terms of drilling time and 



456 Subsurface Geologic Methods 

drilling rate, each of which has its own specific uses and limitations. The 
relationship between the two and their differences are discussed. Drilling- 
time data may be applied to many engineering and drilling problems and 
have proved of considerable value to contractors, drilling crews, and 
operators. The multiple uses to the geologist involve general correlation 
problems and detailed studies of lithology. The geologic application of 
drilling-time data is an additional technique of particular value in the 
determination of potentially productive sections of a well bore and in 
the calculation of recoverable reserves. 

Many methods have been used to measure and record drilling time. 
A technique used by the author is described and illustrated in which 
mechanical recording of depth in reference to elapsed time is translated 
into graphs or logs, which may be used in the solution of a great many 
geologic and engineering problems. The amount of section so logged and 
the scale employed are determined by the requirements of the individual 
problem. Detailed instructions are included whereby a geologist not ex- 
perienced in the use of this technique may learn how to prepare and in- 
terpret drilling-time logs in any area in which he may be interested. 

Definitions 

Early methods of determining rate of penetration were crude and 
approximate and generally consisted in recording the time required to 
drill a certain number of feet of hole or the number of feet drilled per 
hour. Data thus acquired may be adequate for some purposes, but as the 
technique of using drilling time became more widely employed, certain 
advantages were observed in determining the specific net amount of time 
required to drill each foot. Before discussing the application of drilling- 
time data, the distinction should be understood between drilling rate and 
drilling time. 

By definition, rate of penetration is a fixed lithologic property, even 
though it may be diagnostic only when used as a relative term. Drilling 
time is the duration of time required in the actual drilling of a unit of 
depth. Drilling rate is the number of units of depth drilled in a unit of 
time. 

The foregoing may be illustrated by comparison to an automobile 
speedometer. When a car travels a mile in a certain number of minutes 
and seconds, it is a measure of speed comparable to the time occupied 
in the drilling of one foot of formation, which we have defined as drilling 
time. When a speedometer indicator points to 45, it indicates that the car 
is traveling at the rate of 45 miles per hour. This is comparable to rate 
of penetration measured in the number of feet drilled per hour, which is 
the definition given for drilling rate. The distinction is more than academic 
and should be clearly understood, because the interpretation of rate of 
penetration is strongly influenced by the method of recording. 



Subsurface Logging Methods 457 

Drilling Time: Diagnostic of Lithology 

The relationship between rate of penetration and lithology has been 
understood for many years. The application of drilling time was recog- 
nized at least seventy years ago,"^ and the interpretative value has been 
appreciated for more than fifteen years. The use of drilling-time data 
has widened continuously as mechanical devices for their recording have 
become available and new applications of the data have been found. 

Drillers probably were the first to learn that a change in drilling 
rate meant a change in the type of rock penetrated by the bit. Limestone, 
cap rock, shale, or sandstone would be recognized prior to confirmation 
by examination of the cuttings. Until recently the application of drilling- 
time data was essentially one of qualitative significance, and methods of 
observing rate of penetration were formerly far from exact. 

Regardless of the method of observation or the crudeness of technique 
in measuring drilling time, one fact remains unchanged and should be 
emphasized at the outset. Drilling-time characteristics are but one of 
many diagnostic properties of a rock; therefore, the use of rate-of -pene- 
tration data must be considered as corroborative of other ■ techniques by 
which lithologic properties are recognized. Sample examination, coring, 
electric and radioactivity logging, temperature and caliper surveying, 
drilling-time logging, and other means of geologic observation must go 
hand in hand to meet today's demand for more scientific methods for 
finding oil and gas reserves. 

If, when drilling under uniform conditions, the bit's penetration 
changes from a slow rate to a faster rate or vice versa, it is an indication 
that a new type of lithology has been encountered. The obvious examples 
are readily recognizable and well known. A driller could hardly fail to 
know when he has encountered cap rock or a sandstone by "the way the 
bit acts." Certainly a change from crystalline limestone to dense dolo- 
mite or from hard shale to limestone is more difficult to observe, but any 
change in the characteristics of lithology should cause a change in the 
rate of penetration, provided all contributing factors remain constant. 

One might question whether rate of penetration is scientifically a 
property of a rock because, at the present time at least, it is not capable 
of being catalogued in quantitative terms. This is a weakness in technical 
procedures, or the fault may be the inability to evaluate contributing 
factors, but this does not alter the fact that rate of penetration is a petro- 
graphic property. If there were no means of determining the identity 
of mineral constituents of a rock, it would not be wrong to state that 
mineral composition is a diagnostic property by means of which a specific 
lithologic type could be identified. 

It can not be claimed that a certain sandstone having ninety percent 
quartz, six percent feldspar, three percent femags, and one percent auxil- 
iary minerals, for example, will drill at a rate of one foot in three minutes 

Carll, J. F., Discussion of Driving Time: Second Geological Survey of Pennsylvania, vol. 3, 1880. 
By personal correspondence witli Dr. J. V. Howell. 



458 Subsurface Geologic Methods 

and fifteen seconds; nor, conversely, that any rock which drills at that 
rate is necessarily that particular type of sandstone. Under one set of 
conditions it may drill in exactly that amount of time, and with other 
drilling conditions it may require much less or much more time. Never- 
theless, we are defining rate of penetration as a fixed lithologic property, 
comparable to electric, radioactive, or mineralogic properties, and the 
hypothetical fixed time required to drill one foot of sandstone such as 
that described above could be defined as diagnostic of that rock. 

It is hoped that some procedure for quantitatively evaluating contrib- 
uting factors will enable drilling-time properties to be understood as fixed 
characteristics after allowing for the amounts of time required in the 
drilling of a unit of depth that are not attributed to the inherent lithology 
of the rock. Among the contributing factors referred to are the size of 
the hole, the type of bit, the drilling weight employed, the rotary speed, 
torque and friction, and the condition of the mud. This subject is worthy 
of study as a research project in order to determine the net-drilling-time 
value of a formation having uniform characteristics over an area large 
enough that adequate drilling-time data could be accumulated and studied. 

The qualitative interpretative value of drilling-time data, however, is 
not impaired by the absence of quantitative calculations. Observations by 
the author in the drilling of hundreds of thousands of feet of hole have 
shown that drilling conditions are insignificant in comparison to lithology 
in determining the rate of penetration of a rock formation. Obvious ex- 
ceptions have been noted, but the foregoing observation holds true. It 
has been shown in many instances that a change in formation will be re- 
flected by a change in drilling time even when very dull bits have been in 
use or where other conditions would be expected to obliterate any evi- 
dences of change in drilling time. Perhaps the condition that affects drill- 
ing time more than any other is holding up on drilling weight as when 
straightening a hole tending to deviate. Under such disadvantageous 
conditions, there may be no pronounced change in the actual time required 
to drill a unit of depth when passing from one lithology into another, 
but the pattern of the curve plotted from drilling-time data seldom fails 
to reflect the change in lithology. Adverse drilling conditions do require 
more careful interpretation than favorable drilling conditions, but the 
effect of changes in lithology is seldom completely obscured. 

Because drilling time is a qualitative property of a rock, it is impor- 
tant that correct identification of lithologies be based on the observation 
of relative values. A foot of hole that is drilled in five minutes at one 
depth may be interpreted as a sandstone, and another foot drilled under 
conditions differing from the first-mentioned foot requiring also five min- 
utes for drilling may be interpreted as a shale. In each case the interpreta- 
tion is based on the relative time in comparison to previously drilled feet. 
The value of the application of drilling-time data to geologic and engineer- 
ing problems lies in the recognition of this relative interpretation. 



Subsurface Logging Methods 
Drilling Time and Drilling Rate 



459 



The two means of measuring rate of penetration have been defined; 
and, as shown, driUing time is a specific value for each foot, and drilling 




Figure 215. Relationship between electric log and drilling-time data. Black areas on 
electrical log indicate intervals of rapid penetration. 



460 



Subsurface Geologic Methods 



rate is an average value involving the drilling of several feet. The former 
is more exact and is useful where detailed lithologic information is re- 
quired and where difficult correlation problems may be solved only in the 
study of minor features that are best disclosed in the pattern of a log 
plotted from drilling-time data. The latter method requires less time for 
recording original data and for plotting and is useful where rapid interpre- 
tations are required and where it is used in conjunction with other methods 
that are based on average data as well, such as sample-examination logs. 
As will be shown further in this section the pattern revealed by a 




Figure 216. Two curves plotted from data on drilling-time chart of figure 215. 

drilling-time log has characteristics very similar to that of an electric- 
potential curve. The differences between drilling-time and drilling-rate 
logs may be illustrated by the electric log of a well drilled and the original 
drilling-time record of the same well. In figure 215 the porous sands 
3,669-3,673 feet did not drill as fast as the upper and lower sands and may 
the relatively close spacing of the foot marks on the drilling-time chart 
and are confirmed by the electric log. It will be noted that the streak 
3,669-3,673 feet did not drill as fast as the upper and lower sands and may 
be interpreted as a shaly sand. This interpretation is supported by the 
potential curve of the electric log. 

In figure 216 are two curves plotted from the data on the drilling- 
time chart of figure 215, using the normal scale for correlation on the 
left and the enlarged detail scale for lithologic interpretation on the right. 
A close comparison of the detailed curve with the potential curve in 
figure 215 reveals not only the corresponding sand sections but also a 



Subsurface Logging Methods 461 

close resemblance between the patterns of the two curves. Note, for ex- 
ample, such minor features as the slightly sandy shale streaks at 3,666 
feet and 3,691 feet, which are represented as slight bulges in the potential 
curve. The characteristics of electric and drilling-time curves are deter- 
mined by changes in lithology. The drilling-time break at 3,676 feet is 
interpreted as anomalous to lithology because a connection was made at 
this depth and part of this foot was drilled with the clutch out, causing 
an erroneously timed foot to be registered. By marking on the log where 
interruptions in drilling occur, such features may be recognized with ease 
and incorrect interpretations prevented. 

The curves of figure 217, which were plotted from the same data as 
those of figure 216, show by solid lines the loss of detail in using five-foot 
intervals instead of one-foot intervals and by dotted lines the effect of 
averaging when using drilling-rate values. In the upper solid curve on 
the correlation scale the total time for five feet was used and plotted as a 
bar curve according to the manner generally practiced on sample logs. 
In the lower solid curve on the detailed scale, the average time per foot 
was plotted as a point-to-point curve. In both of these curves the presence 
of two sands and one shaly sand is observed, but the exact depths at which 
they occur, their net thickness, and minor lithologic breaks are absent. 
Obviously, it requires more time to plot the curves in figure 216 than in 
figure 217, and the information to be gained is disproportionate to the 
time saved. There is some value in large-interval drilling time and in drill- 
ing-rate curves to be sure, but their use is restricted to problems where 
only general impressions are needed either for correlation or lithologic 
interpretation. In plotting sample logs on the basis of percentage of 
lithologic types present in each sample, the position of major breaks may 
be determined by plotting a drilling-rate curve similar to the upper dotted 
curve in figure 217. Where difficult full-length correlation problems are 
encountered, however, the drilling-time curves of figure 216 will be found 
far more reliable and useful. Drilling-rate logs are useful in sample ex- 
amination work in determining sample lag, but here again the information 
is only exact within the limits of accuracy of the method used. Further 
discussion of the application of drilling time and drilling rate follows at 
the end of the next section. 

Several devices purport to record changes in rate of penetration in 
terms of feet per hour, and mechanical instruments have been marketed 
that provide drilling-time logs or data from which these logs may be 
plotted manually. One of the drawbacks to the use of drilling-time logs 
has been the time required to plot curves of the type illustrated in figure 
216. There is no machine available that will reliably record or plot 
drilling-time logs of this type and eliminate human errors. Considerable 
experimental work along this line has been done, and the need for such 
a device is great. Among the best known to the author is the extensive 
research, laboratory testing, and field demonstrations conducted by Mr. 



462 Subsurface Geologic Methods 

Thomas A. Banning, Jr., of Chicago, Illinois. Under the title "Measuring 
and Recording Various Well Drilling Operations" Mr. Banning has filed 
application for Letters Patent in the United States Patent Ofl&ce. This 
application reviews thoroughly the history of the art of drilling-time re- 
cording and investigates means by which data measurable during the 
drilling of a well can be recorded and plotted to produce logs such as are 
described in this article. Field tests are on record in which remarkable 
accuracy in depth measuring and time coordination were achieved. Such 
equipment, when available, will increase the application of drilling-time 
data as here discussed, and undoubtedly will open up new channels of 
research into lithologic properties hitherto inadequately understood. 

Application of Drilling-Time Logs 

Many of the uses for drilling-time logs have been cited and illus- 
trated in the literature. The value to drillers, contractors, and operators 
is well known. The chief concern is the drilling of the greatest amount of 
hole in the shortest time possible consistent with good safety practices. 
At the same time anyone connected with the drilling of an oil well knows 
that the purpose of drilling a hole is to gain information, the use of which 
may lead to production of oil or gas. The contractor may find drilling- 
time records most useful in the analysis of operations and the study of 
down time as well as pay time. The performance of different types of bits 
in various formations can be observed directly on drilling-time charts 
which reflect the types of formations being penetrated. The driller finds 
drilling-time charts of value as a record of his tour showing exactly how 
much he drilled, what type of formations were encountered and their 
depths, and a record of time down for sundry purposes. The guess work 
is eliminated. But the driller, like the operator, is concerned with finding 
oil, and he knows that reservoir beds are porous media overlain by hard or 
impervious layers. If he knows he is drilling in a section where potentially 
productive formations may occur and has not been given precise instruc- 
tions in respect to coring, logging, or testing, the driller uses his best 
judgment in the interest of the operator. When he observes a drilling- 
time break, he may stop drilling and circulate samples for examination and 
wait for orders before drilling past any formation that might carry oil 
or gas. The drilling-time charts provide an indisputable record of where 
the top of the break was encountered and how many feet have been drilled 
in it. 

The geologic importance of drilling-time logs is evident primarily 
in the fact that foot-by-foot information is available for correlation. As 
has been shown in figures 215 and 216, a drilling-time log, plotted on 
a time scale such that the amplitude between fast and slow peaks gen- 
erally corresponds to the range between the shale base line and the maxi- 
mum peak of an electric-potential curve, provides the geologist with data 
that may be used, within reasonable limits, for much the same purposes 



Subsurface Logging Methods 



463 



as an electric log itself. It is obvious, therefore, that if a drilling-time log 
of a well corresponds in pattern to an electric-potential log of that well 
after the hole has been drilled, the drilling-time log made during the 
drilling could be used for the purposes served by the electric log. This 
has proved true particularly in the Gulf Coast area where long-range or 




Figure 217. Curves plotted from same data as those of figure 216. Solid line rep- 
resents a plotted five-foot interval. Dotted line reflects an average plot. 



464 Subsurface Geologic Methods 

local correlation is based on the succession of a series of beds predomin- 
antly shale and sandstone. The stratigraphic position of any portion of 
a drilling well can be established in advance of electric logging by observ- 
ing the sequence of beds penetrated as revealed by a drilling-time log. 

It would be easy, for example, for the sequence of beds illustrated 
in figure 216 to correspond to a similar sequence of beds above or below 
the portion of the well shown. It would be difficult, if not beyond the 
realm of possibility, for the full length of section drilled and logged, below 
that depth at which drilling time becomes diagnostic of lithology, to 
correspond and be correlated erroneously with the same stratigraphic sec- 
tion of another well where such correlation could otherwise be established. 

The economic and geologic value of this use of drilling-time logs is 
apparent. In the writer's experience many preliminary correlation runs 
of electric logs have been unnecessary because the purpose for which 
they would have been run was adequately served by drilling-time logs 
plotted as the drilling took place. 

The most widely recognized geologic use of drilling time is in con- 
nection with coring operations. Careful recording aids in obtaining ac- 
curacy in well measurements, particularly where continuous coring is 
done over a long section. Drilling time often makes it possible to inter- 
pret the lithology of missing portions of cores recovered and identifies 
the portion of section cored from which the recovered core came. 

Unless 100 percent of the core is recovered, it may be difficult to 
determine the net thickness of productive formations, even with the aid 
of an electric or radioactivity log. In areas where limestone streaks are 
interbedded with saturated sandstone, as in the Oligocene formation of 
south-central Louisiana, it is nearly impossible to interpret an electric 
log correctly without corroborative data. Greatest accuracy may be 
obtained in such problems by the use of electric logs, drilling-time logs, 
and cores combined. 

Another important use of drilling-time data is their aid in the inter- 
pretation of electric logs. It is a common practice in drilling wells in the 
Gulf Coast area to rely on sidewall cores to check questionable shows of 
saturation in beds not cored during the drilling. Some of these question- 
able shows are thin calcareous beds which produce resistivity kicks on 
electric logs that are not unlike those that might be caused by saturated 
sandstones. The detailed examination of an electric log in conjunction 
with an accurate drilling-time log may reveal information on these ques- 
tionable beds sufficient to identify them as calcareous or arenaceous. This 
use of drilling-time logs reduces the cost of side-wall coring and effects a 
further saving of rig time. 

The use of drilling-time logs as an aid in the interpretation of electric 
logs may be applied to the problem of reserves estimate. Estimates of 
ultimate reserves of oil and gas often fail to represent the actual amount 
of eventual recovery. Although great progress has been made in under- 



Subsurface Logging Methods 465 

standing physical and chemical reservoir conditions and factors relating 
to the recovery of petroleum resources, the amount of reserves in place 
is given only as an estimate. Some even discount the value of making 
estimates of this character because of the lack of knowledge or possession 
of empirical data necessary to arrive at reliable conclusions. Efforts are 
continuously being made to increase the accuracy of reserves estimates. 
The oil or gas content of a reservoir bed is generally given in barrels of oil 
or mcf. of gas per acre foot. Lack of adequate knowledge pertaining to 
the reservoir conditions limits the accuracy of the estimate of the for- 
mation's content per unit volume, but the factor given is the best available 
in light of present-day scientific understanding. An estimate of the areal 
extent of a reservoir bed is also subject to considerable latitude because of 
the lack of knowledge concerning migration channels and underground 
drainage conditions. The estimate again is made on the best information 
that can be supplied by the subsurface geologist after mapping the structure 
in which the producing horizon is found. 

In establishing the net effective thickness of the reservoir bed, there 
is a greater means of eliminating the necessity of an estimate, provided 
adequate data are available. In a section where reservoir beds are very 
uniform in respect to lithology, porosity, and permeability, the thickness 
may be determined by the driller's record, an electric log, a radioactivity 
log, or other reliable methods of logging or observing a formation. In 
those reservoir beds where the lithology is not uniform all available means 
may be required to determine the net effective thickness of that bed. Core 
information and analyses, electric and radioactivity logs, and drilling-time 
logs contribute to the best possible answer. As shown in figure 215, the 
less-permeable character of the bed from 3,669 feet to 3,673 feet was in- 
dicated both by drilling-time and electric logs. 

There are many cases where thin shale breaks in a sandstone reser- 
voir or tight calcareous streaks interbedded with saturated sandstone are 
not indicated on electric or radioactivity logs. If recorded on a short 
enough depth interval, however, drilling time will seldom fail to disclose 
the presence and thickness of such breaks. The writer has used drilling 
time recorded at intervals of one-tenth of a foot in a productive section 
where many and very thin impervious streaks were present and only by 
this means was able to determine the exact net effective thickness of the 
reservoir. Therefore, if positive information can be gained as to the 
thickness of a formation, one of the three essential data making up a 
reserves estimate can be assigned a fixed value, and the accuracy of the 
final answer is increased. 

Perhaps the greatest argument for the use of drilling-time logs is their 
value as insurance against the loss of geologic information in the event 
that other types of logging are precluded because of well conditions or in 
case of a junked hole. Because a drilling-time log provides essential data 
corresponding to an electric-potential log, it can be used for correlation 



466 Subsurface Geologic Methods 

to determine the stratigraphic and structural position of a well which 
might otherwise remain unknown. It may also reveal the presence of 
probably porous beds that may be saturated because of their structural 
position, and therefore the economic risk of drilling a new hole or aband- 
oning a location may be substantially reduced. 

Since the publication of the first edition of this symposium, the 
writer has received communication from J. A. Simons, geologist with 
Creole Petroleum Corporation, regarding the use of drilling-time and 
drilling-rate curves in Venezuela. He states, "This technique (drilling-time 
logging) is extremely useful in this area (Pedernales District) and is used 
as a method of bottoming a field well at the base of a known productive 
sand lense and to prevent the penetration of the next lense, known to be 
salt-water bearing ... It developed that the plotting of drilling time in 
minutes per foot had been tried and poor results were obtained. A different 
method of plotting — feet per hour per one foot — was tried and has been 
adopted as a standard practice." 

Simons states further, "If it requires a certain number of minutes to 
drill one foot, that is the drilling time for that foot. But that foot was also 
drilled at a certain rate which can be expressed in units of depth per units 
of time . . . When drilling time is plotted in feet per hour for one-foot 
intervals, a semi-logarithmic form of curve is obtained which dampens the 
effect of very slow feet caused by harder streaks or by the inattention 
of the driller, and which conversely exaggerates the effect of fast drilling 
that cannot be caused by anything but the bit entering a zone of easier dig- 
ging. The scale is chosen to fit the fastest drilling observed, and fluctua- 
tions in the hardness of shale do not cause a widely varying curve, and the 
S.P. log in the shale section is more closely approximated." 

The above discussion is illustrated in figure 218 and is introduced 
in this paper as an illustration of individual adoption of variations in 
selection of scale and method of plotting. It should be pointed out, how- 
ever, that drilling-rate values cannot be determined without first measur- 
ing drilling-time values and it would appear that calculating the reciprocal 
values is an unnecessary step. The use of the zero base line at the right 
and the plotting of reciprocal values, even on a straight arithmetic basis, 
might have advantages in the analysis of special problems. 

Commenting on the above, Mr. T. A. Banning, Jr., Chicago, Illinois, 
writes*: "The answer to the foregoing question (drilling time or drilling 
rate — which?) must depend on the use to which the information given is 
to be put. For some purposes time will be the most important factor of 
the ratio — for other purposes rate will be the important factor. In any case 
drilling time and drilling rate are mathematically interchangeable, and 
under the conditions presented in the case of well drilling these data are in 
all cases sufficient to permit presentation of the ratio either way." 



* Personal communication. 



468 Subsurface Geologic Methods 

This fact may be illustrated further by the following mathematical 

relationships: 

distance 

rate = : 

time 

distance = rate X time; or 

distance = feet per hour X hours 

distance 



time = 



rate 



If distance = 1, then rate = — or, conversely, time 



time rate 

It is important to recognize the significance of using unit-depth in- 
tervals in these calculations. If, by any mathematical means, drilling rate 
is calculated from drilling-time data, the rate represents only the average 
speed of penetration for the depth interval used. Consequently, drilling 
rate obscures any minor variations that may occur within the depth interval 
used. As has been mentioned previously, some experimental work has been 
done by Mr. Banning and the writer in the recording of drilling time on 
one-tenth foot intervals. Correlation with complete core recoveries ol 
sections thus logged showed a definite and important relationship between 
lithology and drilling time. If, in this drilling-time logging, full foot in- 
tervals had been used and plotted as drilling rate, the resulting average- 
value log would fail to reflect these lithologic details. Therefore, the pur- 
pose to be served will determine the depth intervals to be used and the 
character of the curve to be plotted. 

Method of Preparing Drilling-Time Logs 

The experience of the writer in the use of drilling-time data has been 
based on records obtained from "Geolograph" charts for the most part. 
Although there are other means by which usable data may be collected, 
perhaps the most practical source of complete drilling-time records is the 
"Geolograph." For this reason it is considered in place here to describe in 
detail the technique recommended in translating the original record into 
the drilling-time log for which multiple uses have been described. 

It is unnecessary to include the maintenance of the equipment, which 
is the responsibility of the service company. A few points should be 
kept in mind, however, by the geologist desiring to obtain as perfect records 
as possible. A drilling-time log is the plotted curve of two components, 
time and depth, each requiring accuracy within the limits of observational 
errors. The "Geolograph" machine provides two inking pens, generally 
supplied with inks of different colors. One pen indicates by vertical and 
horizontal lines the amount of time that drilling is in progress and when 
it has been stopped or when the bit is off bottom. The other indicates by a 



Subsurface Geologic Methods 



G-8 

OPESATOB A. B.C. 


Geolograph Chart 
OIL CO. 




HO 1 




»" FEE 




t-oc, CTR. 




SEC 1 T 1 N. P 


1 E 


COUNTY XYZ 




STATE ANY 




oATc ON: 2 - 24- 49 


T. t>. ON: 5996' 




riMEON- tSoAM 
a S P M 




TO. OFF: 6124' 






Plate 10. "Geolograph" chart showing essential data 
used in preparing a drilling-time log. Displace- 
ment of red line to right marks every ten feet 
of section penetrated. 



Subsurface Logging Methods 469 

slanting stroke the completion of exactly one foot drilled, or two feet if 
set for recording on two-foot intervals. Therefore, the exact depth marked 
by each stroke of the pen must be known and the length of time occupied 
in the drilling must be determined. Plate 10 is a "Geolograph" chart show- 
ing essential data used in making a drilling-time log. 

It is strongly recommended that the geologist using complete drilling- 
time data keep his own pipe tally. Practices differ among contractors in 
keeping pipe tallies, but inasmuch as the object here is to know the depth 
indicated by each mark on the chart, the geologist's tally must show all 
pipe in the hole at the time the mark is made. This becomes most important 
when changes in the drilling string are made, as for coring. The tally 
should show first the bit, subs, and drill collars in the string, followed by 
each joint of drill pipe added. The kelly should be measured accurately 
and its length recorded. The geologist should also observe whether it is 
the practice of the driller to drill the kelly down or make his connections 
with some length up on the kelly. The "Geolograph" automatically shows 
when a new joint of pipe is added, and the geologist should write on the 
chart the kelly-down depth of each connection, accurate to the nearest 
hundredth of a foot, as shown on his pipe tally. (See connections at 
6,017.27 feet and 6,048.61 feet in pi. 10) . 

The charts are generally changed twice a day and when received for 
translating will show the date, time of chart change, depth of beginning 
and end of each chart, and correct depth at each connection or begin- 
ning of a round trip. The depth of each mark or every fifth mark be- 
tween connections should then be written on the chart, as 6,000 feet, 
6,005 feet, 6,010 feet, et seq., plate 10. If the drillers have been 
very careful in throwing in the clutch at the proper time, the number 
of marks should be identical with the number of feet drilled. Often 
this is not the case, and observation of drilling operations and experi- 
ence with how such discrepancies occur will show the geologist where 
errors take place. The most common error of this type is made by 
the driller in failing to throw in the clutch after making a connection at 
exactly the same depth as when it was thrown out prior to making the 
connection. Changes in drilling weight caused by settling out of rock cut- 
tings may cause what might be called "false drilling" or the redrilling of 
depth without drilling a new formation. Or, if the driller fails to throw in 
the clutch as soon as the bit reaches bottom with the same weight as when 
it left bottom, some new hole may be drilled without being recorded. 
(See 3,676 feet in figs. 215, 216.) Errors of these kinds are readily 
understood when one realizes that the drilling crew is most busily occu- 
pied when connections are made. Often the correction for depth will be 
made immediately after a connection, but the geologist must use his best 
judgment in making all corrections so that when a depth is assigned to 
a mark it will be correct. 

Where no "extra" marks have been made and no "skips" noticed, 



470 Subsurface Geologic Methods 

there may accumulate fractional-foot errors, which may be designated as 
"creep." Over long-continued drilling, involving several connections or 
even round trips, a correction may be required, and it is difficult to know 
where it should be made. Having written the kelly-down depth at each 
connection and the correct depth at a round trip, accurate in each case 
to the nearest hundredth of a foot, k will be obvious what depth should 
be assigned just before or just after making a connection. For example, 
if a connection has been made at 6,079.04 feet in figure 219 and a foot 
mark is shown immediately after the connection, it would be reasonable 
to indicate the depth of the mark prior to the connection as 6,078 feet, 
as the total number of feet drilled corresponds to the number of foot 
marks on the chart. Had the connection been made at 6,078.97 feet, 
however, and a foot mark shown just prior to making the connection, 
with a normal drilling interval following the connection before the next 
foot mark was recorded, it would be reasonable to assign a depth of 
6,079 feet to the foot mark prior to the connection. Apparent errors of 
this type would be the result of "creep." 

The foregoing is only suggestive of some generalizations that may be 
made in preparing the chart for time determinations. Frequent use of 
drilling-time charts will increase the speed and accuracy in obtaining 
proper depth designations. The chart reproduced in plate 10 provides 24 
inches for recording twelve hours of time, or two inches per hour. The 
hour is divided into twelve divisions of five minutes each, and these are 
further divided into one-minute divisions. For high speed, when the re- 
quirements demand it at the sacrifice of accuracy, the eye can measure the 
distance between foot marks within an accuracy of approximately twenty 
to thirty percent. Many uses of drilling-time logs require greater accuracy 
than this, however, particularly when minor breaks in the over-all pattern 
are used to correlate with minor breaks on an electric-potential log. To 
obtain greater accuracy, with an error of less than one percent, measure- 
ments may be made with an engineer's scale, using the thirty-divisions-per- 
inch scale for the purpose. This scale, placed on the chart having two 
inches per hour, will provide sixty divisions per hour or one per minute. 

The use of a printed form to tabulate the time readings may be con- 
sidered as an extra step, and some may suggest plotting directly from 
observation of the chart. It has been found that this extra step not only 
avoids many errors that might otherwise be made but actually saves time. 
In addition one often wishes to plot the same data on more than one scale, 
in which case the time saving is considerable. The same person may make 
the readings and write the tabulation, but if one person keeps his eye on 
the scale and chart and another writes down the time on the form, it will 
save two-thirds of the time required for this operation. Additional time 
can be saved in the following step, plotting the log from the tabulation, 
by having one person read the units off as another plots the coordinates. 



Subsurface Logging Methods 471 

Figure 219 illustrates a type of form on which drilling-time data may be 
tabulated. 

Determination of the time factor from the chart, using the engineer's 
scale, is made by placing the scale parallel with the long dimension of 
the chart and at the left of the base line from which the slanting stroke is 
made. The zero of the scale is then placed at the top side of the depth 
stroke and the position of the top side of the succeeding depth stroke noted 
on the scale. The observer can read the scale to the nearest quarter of a 
division or fifteen seconds of time, and this reading is the time factor for 
the foot being measured. Because of the type of coordinate paper recom- 
mended for plotting the log, it will facilitate recording and plotting these 
time factors if decimals instead of fractions are used, as in figure 219. 

No difficulty will be encountered in thus reading and tabulating the 
time where there have been no interruptions in drilling. If drilling has 
been stopped at other than at the completion of an even foot, as at 6,021- 
6,022 feet in plate 10, the time out must be subtracted from the total time 
to indicate only the net time consumed in the drilling of the foot. The 
position of the 6,022-foot stroke on the scale is shifted to the top of the 
bottom horizontal line, indicating the end of the time out. The position 
on the scale of the top side of